Materials Included In Kit

Additional Materials Required

Prelab Preparation

Experiment 5: Color

1. Cut out the color wheels and Benham’s disks before class.
2. Poke a hole in the center of each disk with the pin. Make sure the hole is not larger than the head of the pin.

Prism—Teacher Demonstration 

1. Obtain a flashlight and the prism. 
2. Stand the prism on its end so that it sits vertically on a flat tabletop.
3. Place the flashlight right next to the prism, flat on the tabletop, pointing toward a distant wall or overhead projector screen.
4. Shine the light from the flashlight through the prism and allow the light to project onto the distant wall or overhead projector screen.
5. Rotate the prism until the rainbow spectrum appears. Typically, to form a bright spectrum, one of the pointed edges of the prism will point towards the flashlight at a slight angle.
6. Once the appropriate orientation of the flashlight and prism is determined, rotate each piece as necessary to project the rainbow spectrum on the desired surface. Typically, the farther away the projection wall is from the prism, the larger the spectrum will be for students to observe.
7. Turn off the classroom lights, if necessary for students to see the spectrum, and allow students to record their observations in the Color Worksheet.

Safety Precautions

Handle the pins with care. Do not look directly into the flashlight with or without a lens. Do not look directly at the sun through the polarizer or prism. Please follow all normal laboratory safety guidelines. Remind students to wash hands thoroughly with soap and water before leaving the lab.

Disposal

The materials from each lab may be saved and stored in their original containers for future use.

Lab Hints

Experiment 1: Mirrors and Reflection

• Enough materials are provided in this kit for four groups of students to work at the same lab station. This laboratory activity can reasonably be completed in 30 minutes.
• Prepare copies of the protractor sheet for each lab setup.

Experiment 2: Refraction

• Enough materials are provided in this kit for four groups of students to work at the same lab station. This laboratory activity can reasonably be completed in 30 minutes.
• When light enters the semicircular dish at the midpoint of the flat side (the center of the complete circle), the refracted ray will strike the curved end of the dish “normal” to the surface (see Figure 15). Therefore, there will be no refraction of light when the light beam exits the water at the curved surface. The angle of the light beam traveling in the water will be the same as the light beam that exits the dish through the curved side and seen on the protractor sheet. This is why it is important to look through the dish at the center of the flat side of the semicircular dish.
  {13551_Hints_Figure_15}

Experiment 3: Thin Lenses

• Enough materials are provided in this kit for four groups of students to work at the same lab station. This laboratory activity can reasonably be completed in 30 minutes.
• To show that the eyepiece lens of the two-lens system focuses on a real image, the student can shine a flashlight through the objective lens (15 cm convex) so that the pin’s shadow becomes the real image. The student can use a piece of lens paper to see the real image (shadow) of the pin on the light-exiting side of the lens. The shadow should be visible on the backside of the lens paper as well. The student can then carefully use the convex lens to magnify the real image and see that it looks about the same size as the magnified pin in the two-lens system. Make sure the filter paper is large enough to block stray light from the flashlight. Caution students not to look directly at the flashlight with the convex lens.
• Caution: Remind students to turn off the flashlight when they look through the lens system (or concave lens) to view the images.
• Students may not understand why a concave eyepiece lens can act as a simple magnifier since it does not produce a magnified image under normal viewing conditions. However, a concave lens can act as a simple magnifier when it magnifies a virtual object. A virtual object is an “object” located on the transmission side of the focusing lens. This object does not actually exist. In the focused terrestrial telescope, the objective lens will focus the incident light to its back focal point. However, the eyepiece lens is positioned in front of this point, between the objective lens and its back focal point. Therefore, the focused light from the objective lens will be bent by the eyepiece lens before it can actually form a real image. The final image produced by the eyepiece lens will be located and oriented as if the viewed object has a negative distance (according to the thin-lens equation), meaning it is located on the transmission side of the eyepiece lens. The object is virtual, and the optics allow the concave lens to act as a simple magnifier in this situation.

Experiment 4: Polarization

• Enough materials are provided in this kit for two groups of students to work at the same lab station. This laboratory activity can reasonably be completed in 30 minutes.
• Plastic zipper-lock bags also show stress points around the zipper-lock region. Cellophane tape also shows interesting colors between the crossed polarizers. Clear packaging tape works well; transparent Scotch^® tape will not work. Materials, such as the window from an envelope, a clear plastic deli tray or a transparency sheet, may also show various effects of rotating plane polarized light.
• Try a clear plastic protractor or ruler between the crossed polarizers to examine the effect that these objects have on polarized light. Drill a hole or hit the object with a hammer and observe new stress points created in the object.
• The phenomenon of polarization can be demonstrated with a Slinky^® and two textbooks. Have two students each hold a textbook perpendicular to a table or floor and parallel to the other. The books should be slightly farther apart than the width of the Slinky. Stretch the Slinky between the two books and allow a student to hold each end. Oscillate the Slinky up and down and observe that the transverse wave that is created will travel past the books to the other end of the Slinky. Now, oscillate the Slinky from side to side and notice that the transverse wave does not continue past the books. The textbooks represent the slits in the polarizing film and the Slinky represents the transverse light waves. Try the Slinky demonstration with longitudinal or compression waves to show the students that these waves cannot be polarized.
• The Kaliedoscoptical Activity (Flinn Catalog No. AP8781) is a great polarization demonstration for showing how polarized light can be rotated by certain types of chemicals (chiral molecules).
• More polarized light demonstration activities are provided in the Basic Polarized Light Demonstration Kit (Flinn Catalog No. AP9307).

Experiment 5: Color

• Enough materials are provided in this kit for two groups of students to work at the same lab station. This laboratory activity can reasonably be completed in 30 minutes.
• The prism experiment is set up as an optional demonstration due to the need for a very dark room, and the need to project the light onto a distant wall. This may be difficult to do in a busy laboratory. Perform this quick demonstration the day before or just before beginning the lab as an introductory experiment into the world of light and color.
• The colors perceived during the color experiments will depend on the light source being used. Not all light sources (i.e., flashlights or fluorescent lights) emit all the colors of white light with the same intensity. Natural sunlight will display “true” colors, but it may not be convenient or safe for your classroom environment. Students should not look directly at the sun at any time.
• Students need to share the dry erase marker and the color filter paddles.
• The green color tends to be the most intense so filters crossed with the green filter tend to produce colors with more of a green hue to them.
• The larger the Benham’s disk, the higher the resolution will be between the observed colors of the rings. To demonstrate this feature, enlarge the Benham’s disk using a copy machine.
• Longer and thinner lines produce more brilliant colors on the spinning Benham’s disk. Green and red/orange appear the brightest. Violet and yellow are more difficult to observe. Students can experiment with the thickness and style of the lines to discover any differences in the color brightness or color patterns. Try colored dry erase markers and see if this affects the pattern of the spinning Benham’s disk.
• Color and Light—Spectrum Demonstration (Flinn Catalog No. AP6172) is an excellent demonstration for projecting a large spectrum of colors and for showing color mixing.

Teacher Tips

• Set up each lab station accordingly before class. Students should leave the stations as they find them before they move on to the next lab station.
• Before class, prepare copies of the student worksheets for each student. The Background information for each experiment can also be copied at the instructor’s discretion.
• Prepare 8–10 copies of the Protractor Sheet to be used at various lab stations.
• Most of these experiments work best in a darkened classroom. However, make sure the room is not so dark that it prevents students from seeing as they walk through the classroom to the different lab stations as they complete the experiments.
• Students should refer to their physics or physical science textbooks for further information regarding the reflection, refraction, the thin-lens equation, Snell’s law, polarization and/or color.

Further Extensions

Experiement 3: Thin Lenses

Students can make a microscope by reversing the objective and eyepiece lenses. The objective lens of a microscope is a short focal length convex lens, and the eyepiece is a long focal length convex lens. The object should be placed just beyond the focal point of the objective lens. Spherical aberration is very apparent when looking through the lenses in the microscope configuration and it is sometimes difficult for students to locate the object and bring it into focus.

Sample Data

Experiment 1: Mirrors and Reflection

Cork position: ___40º Quadrant 4___
Real object angle: ___40º Quadrant 2___
Eye position: ___40º Quadrant 3___

Real object appearance compared to reflection: The object behind the mirror appears to be about the same size as the reflection in the mirror.

Cork position: ___15º Quadrant 4___
Real object angle: ___15º Quadrant 2___
Eye position: ___15º Quadrant 3___

Real object appearance compared to reflection: The object behind the mirror appears to be about the same size as the reflection in the mirror.

Cork position: ___30º Quadrant 4___
Real object angle: ___30º Quadrant 2___
Eye position: ___30º Quadrant 3___

Real object appearance compared to reflection: The object behind the mirror appears to be about the same size as the reflection in the mirror.

Experiment 2: Refraction

Observations

Air to Water: Just the center line is seen through the water. The pin and cork are not visible when they are placed at the 50° position.

Water to Air: The pin and cork are visible to the left of the vertical line when they are positioned at the 50° position.

Experiment 3: Thin Lenses

One-Lens System Observations

15-cm convex lens: The projected image (shadow) of the pin and cork is upsidedown. The image shadow is smaller than the object but it comes into sharp focus on the card.

5-cm convex lens: The projected image (shadow) of the pin and cork is upsidedown. The image shadow is much smaller than the object and comes into sharp focus on the card, surrounded by a point of light.

5-cm concave lens: No image focuses on the card at any position. When looking into the lens (with the flashlight turned off), the image of the pin and cork (not a shadow) is smaller compared to the object and is upright.

Two-Lens System Observations

Convex lens: The image is upsidedown and about two to three times larger than the size of the object when the object is viewed at the same distance away (95 cm). The image of the object is not complete. The top and bottom of the pin and cork have been cut off in the image. The image is very bright and clear and fills the entire eyepiece lens.

Concave lens: The image is upright and is about the same size as the image viewed using the 5-cm convex lens. The image of the object is not complete. The top and bottom of the pin and cork have been cut off in the image. The image actually appears to come into view in the objective lens instead of in the eyepiece, so the image of the pin and cork does not fill the eyepiece lens. The image of the pin and cork in the objective lens is very bright and clear, while the viewed region around the objective lens is blurry. There is less distortion in the image of the pin and cork compared to the 5-cm convex lens, but the image takes up less space in the total eyepiece viewing area.

Experiment 4: Polarization

Observations

Single polarizer: The light coming through the single polarizer is not as bright. Some of the light is absorbed.

Two polarizers: The light coming through the two polarizers is darker than the single polarizer. As one polarizer is rotated, the brightness of the light coming through diminishes. When the polarizer has rotated 90°, all the light is blocked and the polarizer looks black. As the polarizer rotates beyond 90°, light begins to come through the polarizers again and reaches a maximum after turning the polarizer 180°. Then the light begins to fade again, and the polarizers appear black after 270° of rotation. The maximum light is transmitted again when the polarizer has made a complete circle.

Two Crossed Polarizers and Plastic Bag: The two crossed polarizers block all the light and appear black. When the plastic bag is inserted between the crossed polarizers, light is transmitted again. When the bag is square with the orientation of the polarizers (either horizontally or vertically), no light is transmitted. When the bag is at an angle with the crossed polarizers, light is transmitted through the polarizers. The peak intensity transmitted through the polarizers is when the plastic bag is at a 45° angle with respect to the polarizers. The color of the light transmitted is the same as before—white light.

Two Crossed Polarizers and Plasticware:  The plasticware shows a rainbow of colors when placed between two crossed polarizers. The color is always present no matter what angle the plasticware is at compared to the polarizers, meaning light is always transmitted by the plasticware. However, the color and intensity do change as the plasticware, or polarizing film, is rotated. But color is always present. The light is never completely blocked by the crossed polarizers.

Experiment 5: Color

Prism—Teacher Demonstration

Observations

The white light from the flashlight turned into the colors of the rainbow when it transmitted through the prism.

Analysis

Explain why white light “changed” into rainbow colors after transmitting through a prism.

The white light changed to colors due to dispersion and refraction. The index of refraction of the prism material (glass or acrylic) varies with the wavelength of light. Therefore, each wavelength (color) is refracted as it travels through the prism by a different amount. This separates white light into its colors.

What would happen if the rainbow colors projected by the prism entered another prism?

If the rainbow of colors travel into a new prism, the colors may bend further apart, or they may bend closer together forming white light as they exit the prism. It would depend on the orientation of the prism.

Color Wheels

Observations

When spun, the blue and green color wheel appeared to be cyan (grayish blue), the red and green color wheel appeared to be a mustard yellow, and the red and blue color wheel appeared to be magenta (cranberry).

Analysis

Explain how the color wheels display the color that is seen when they are stationary and when they are spinning.

See Background information for acceptable answers.

Color Filters Analysis

By what process—color mixing by subtraction or color mixing by addition—do the color filters transmit the color of light that is visible?

The colors mix by subtraction because some light is absorbed as it travels through the filter. The transmitted light is what is observed (i.e., a green filter allows green-wavelength light to pass through and absorbs the other color wavelengths).

Compare the resultant colors from the experiment to those in the color wheel (see Figure 8 in the Background section).

The resulted colors produced when two or more color filters were crossed were the expected colors according to the color wheel. Red and yellow colors blend to produce orange, while blue and yellow colors blend to produce green, etc. When the primary colors of paint were mixed (red, blue and yellow), the light appeared to have no color. In other words, the light transmitted through the three filters was “dirty” white light or a combination of all colors.

Benham’s Disk

Observations

Initial colors on the Benham’s disk: Black and white

Appearance of clockwise spinning Benham’s disk (Draw a picture to illustrate any observed pattern. Use colored pencils if desired.):

The lines on the spinning disk appear to change into the colors of the rainbow. The colors disappear when the disk stops. Violet is on the outside and red is on the inside. It is more difficult to observe the red color than the violet color. Green is very distinguishable.

Appearance of counterclockwise spinning Benham’s disk (Draw a picture to illustrate any observed pattern. Use colored pencils if desired.):

The lines on the spinning disk again appear to change into the colors of the rainbow. However, the color pattern is reversed. Now, violet is on the inside and red is on the outside. It is more difficult to observe the violet color now, compared to the red color. The green color is still very visible.

Analysis

Develop a hypothesis explaining the change in appearance of the disk when rotated clockwise and counterclockwise.

The color change could be the result of the difference in the speed of the lines on the disk. The inside lines will travel slower than the outside lines. Our eyes perceive the colors due to the different rotational speeds. The different locations of the lines determine the colors that are observed. The red arc is created by the first line in the rotating series. The violet arc is created by the last line in the rotating series. When the rotation is reversed the colors reverse order because the last line (violet) is now the first line and now appears red.

Answers to Questions

Experiment 1: Mirrors and Reflection

1. What is the relationship between the apparent size of the object when positioned behind the mirror at the apparent location of the image and the size of the reflected image?

   The apparent size of the object behind the mirror and the size of the reflected image are nearly the same.

2. Formulate a statement that describes the direction that light reflects off a flat mirror.

   Light reflects off the flat mirror at the same angle, with respect to the perpendicular normal line, as the incident angle. Light incident on the flat mirror at a 40° angle from the normal line will reflect off the mirror at a 40° angle.

3. What type of image does a flat mirror form—virtual or real? How do you know?

   It is a virtual image because the image does not form on a screen and can only be seen when looking into the mirror.

4. What is the minimum height a flat mirror must be in order for a person to view his or her feet when the mirror stands vertically?

   In order to view one’s feet using a vertical flat mirror, the mirror must be at least half the height of an individual (assuming the head and feet are equidistant from the midpoint of the body).

Experiement 2: Refraction

1. What happens to the direction of the light rays when they pass into different media (i.e., from air to water, or water to air)?

   The light rays are bent. When traveling from air into water, the light rays bend towards the normal line. When traveling from water into air, the light rays bend away from the normal line.

2. Use the data and Equation 2 from the Background section to calculate the index of refraction of water. Assume that the index of refraction of air is equal to 1.0.

   (1.0) x (sin 70°) = n_w x (sin 45°)
   n_w = (sin 70°)/(sin 45°) = 1.3

3. When light passes from a medium with a high index of refraction into a medium with a lower index of refraction, which direction does the light bend?

   The light bends away from the normal line resulting in a larger angle compared to the incident angle.

4. When trying to catch a fish, would a bird (such as a pelican) dive into the water horizontally in front of or behind the image the fish it sees? Explain. Draw a picture if necessary.

   The bird would dive into the water behind the image of the fish it sees because the light transmitted from the water into the air (the light reflected off the fish) bends away from the normal as it enters the air, making it appear as if the fish is closer to the surface and further ahead of the bird than it really is.

Experiment 3: Thin Lenses

1. Use the data and Equation 1 from the Background to calculate the experimental focal length of each lens. Remember that distances are measured from the lens in centimeters.

   15-cm convex lens
   1/f = 1/50 cm + 1/22 cm
   f = 1/0.0655 = 15 cm
   
   5-cm convex lens
   1/f = 1/35 cm + 1/6 cm
   f = 1/0.195 = 5 cm
   
   5-cm concave lens
   Focal length cannot be determined because a real image was not formed to measure the image distance.

2. How is the image orientated for the convex lens?

   The convex lenses form upsidedown images in this experiment.

3. When the two lens where used in combination, what type of instrument was created?

   This instrument is a simple telescope.

4. Where was the 5-cm concave lens positioned in relation to the focal point of the objective lens when the image was in focus?

   The 5-cm concave lens was placed closer to the objective lens than the focal point of the objective lens. It was placed approximately 5 cm in front of the objective lens’ focal point, meaning the transmission-side focal point of the concave lens nearly aligns with the transmission-side focal point of the objective lens to form the clear, enlarged image.

5. How many lenses are actually being used to see the image produced by the two-lens system? Three lenses are used—the objective, the eyepiece and the lens in the eye.
6. Is the image that is viewed in the two-lens system virtual or real? Explain. The image appears to focus on the incoming-light side of the lens, and therefore it must be a virtual image. The eyepiece lens acts as a simple magnifier when it projects an enlarged virtual image that the eye sees as an object. Real images will actually form on a screen on the transmitting side of a lens. Real images cannot be seen when looking through a lens.

Experiment 4: Polarization

1. What is the definition of polarization? A process or state in which rays of light exhibit different properties in different directions, especially the state in which all the vibration takes place in one plane.
2. How does an analyzer work? An analyzer is the second of two polarizing filters in the path of light that is used to determine the orientation of the polarized light from the first polarizing filter. The polarized light from the polarizer is absorbed by the analyzer if it is in a perpendicular orientation and passes through if it is parallel to the first filter.
3. Why might the plasticware, when viewed through an analyzer change color? The plasticware changes color when viewed through an analyzer because the plastic is an optically active material that changes the angle of the plane of polarization as polarized light passes through it. Each wavelength (color) of white light is rotated by a different angle compared to the initial polarization plane, so only the wavelength (color) of light that rotates to the orientation of the second polarizer (the analyzer) will pass through and be seen.

Teacher Handouts

13551_Teacher1.pdf

Introduction

This all-in-one Optics and Light Kit is designed to provide the opportunity to explore the fundamental properties of reflection, refraction, color mixing and polarization. The four hands-on lab stations offer experiments on different aspects of optics and light.

Concepts

• Law of reflection
• Flat mirrors
• Law of refraction
• Snell’s law
• Index of refraction
• Concave lens
• Thin-lens equation
• Convex lens
• Telescopes
• Polarization
• Color mixing by addition
• Perception of color
• Newton’s color wheel
• Color mixing by subtraction
• Benham’s disk
• Prisms

Background

Experiment 1: Mirrors and Reflection

The reflection from a mirror is very familiar. Mirrors are used nearly every day by people when they comb their hair or drive a car. Light is partially absorbed and partially reflected off every surface. The manner and amount of light that is absorbed or reflected depends on the type of material and the smoothness of the surface. Mirrors reflect almost all of the incoming light, unlike a black shirt which absorbs almost all the incoming light.

Reflected light is governed by one simple principle: the angle of the incident light is equal to the angle of the reflected light, with respect to the normal line at the surface. This is known as the law of reflection (see Figure 1). The law of reflection is easily observable when light reflects off a smooth surface, such as a mirror, a shiny table or even water. This type of reflection is known as a specular reflection. When light encounters a rough, textured surface, however, it will reflect off in many different directions resulting in a diffuse reflection.

The reflected light still follows the law of reflection, but the various reflected angles are the result of the many angles and ridges at the surface that the incident light strikes (see Figure 2). When looking at a reflection from a flat mirror, the reflected image appears to be behind the mirror. The image cannot be seen unless one looks into the mirror. This is known as a virtual image. For a flat mirror, virtual image is upright, the same size and is located at the same distance behind the mirror as the object is located in front of the mirror (see Figure 3). However, the reflected image will be reversed. This is why you often see the word AMBULANCE written AMBULANCE on the front of an ambulance truck. When people look in their rearview mirrors, they can read the word ambulance correctly and respond accordingly.

Experiment 2: Refraction

When light travels from one transparent medium (such as water) into another transparent medium (e.g., glass or air) at an angle with respect to the normal line, the light rays will change direction as they enter the new medium. This is known as refraction. Refraction is the result of the change in the speed of light as the light rays enter different media. Light rays only have the “speed of light,” c, equal to 2.998 x 10⁸ m/s, in a vacuum. In all other transparent media (e.g., air, glass, water), the speed of light is slower than c. The ratio of the speed of light in a vacuum, c, to the speed of light in the medium, v, is known as the index of refraction of the medium, n. Notice that the index of refraction of a vacuum is exactly equal to 1. The index of refraction of air is very close to that of a vacuum with a value of 1.000293. In most cases, the index of refraction of air is simplified to the value of 1.00.

When light rays hit a boundary layer, the incident angle and the angle of refraction are measured with respect to the normal line of the boundary between the two layers (see Figure 4). Willebrord Snell (1580–1626) experimented with light transmitted through different media, and in 1621, he developed the proper relationship between the incident angle and the angle of refraction. The relationship is now known as Snell’s law (Equation 2).

n_i = index of refraction of incident medium
n_r = index of refraction of refracting (transmitting) medium
θ_i = incident angle of light ray (with respect to the normal line) at the media boundary
θ_r = refracted angle of light ray (with respect to the normal line) at the media boundary

Experiment 3: Thin Lenses

The light rays entering a lens are refracted by the lens. The direction the light rays are bent depends on the design of the lens. A convex lens is a lens that bulges at the middle and is also known as a converging lens because it tends to converge light rays to a point on the transmitting side of the lens. A concave lens is thin in the middle and thicker at the edges and it diverges light or bends light away from the center line of the lens. The position of an image formed by a lens can be located by drawing a ray diagram (see Figure 5). Two rules must be followed in order to draw the ray diagram properly. First, the focal point of any lens is the point at which a beam of light converges when it travels parallel to the principle axis of the lens before entering the lens. The reverse is also true: any light originating at the focal point of a lens will interact with the lens in such a way that the transmitted light ray will be parallel to the principle axis. Second, any light ray that travels through the center of a thin lens will not be refracted by the lens but will continue on a straight path.

The thin-lens equation can be used to calculate the position of an image produced by a thin lens, or a combination of thin lenses (Equation 3). Several conventions must be used when applying the thin-lens equation. One convention is that convex lenses have positive focal lengths and concave lenses have negative focal lengths. Two, the focal length is the distance between the center of the lens and the focal point of the lens. Three, light travels from left to right, meaning objects to the left (on the incident side) of a lens, and images formed to the right (on the transmission side) of a lens have positive distances and are real. Real images can be formed on a screen and therefore can be seen by the naked eye. Objects to the right (transmission side) and images formed to the left (incident side) of the focusing lens have negative distances and are called virtual. Unlike a real image, a virtual image can only be seen when looking directly through the lens. It will not form an image on a screen.

f is the focal length of lens
i is the image distance from lens
o is the object distance from lens

A simple magnifier is used to view an enlarged, virtual image of an object that is placed much closer to the eye than the near point. The near point of the eye is the closest distance an object can be placed in front of the eye in which the eye’s lens can still clearly focus the image on the retina. A lot of strain is put on the muscles of the eye when an object is at the near point. Any object positioned closer than the near point will be blurry. For a “normal” eye, the near point is 25 centimeters.

The virtual image formed by the simple magnifier is located an infinite distance away from your eye on the incident side of the lens. This allows your eye to stay relaxed when viewing the clear, enlarged image.

The apparent enlargement of the object depends on the angular magnification of the lens. Angular magnification is measured as the ratio of the angle subtended by the magnified virtual image (θ_i) compared to the angle subtended by the real object (θ_o) when viewing the object at the near point of the eye (see Figure 6). When the lens is held close to the eye, and the object is positioned at the focal point of the lens, the effective angular magnification of a simple magnifier is given by Equation 4.

M_sm is the Angular magnification of a simple magnifier
25 is the the near point for a normal eye (25 cm)
f is the focal length of the magnifying lens

Experiment 4: Polarization

Light (i.e., electromagnetic waves) moves as transverse waves that oscillate in various directions, perpendicular to the direction of the wave motion. Polarizing filters, like those used in Polaroid™ sunglasses, contain closely packed parallel slits that allow only light oscillating in the same plane parallel to the slits to pass through. Light waves oscillating at angles different from the parallel slits are either reflected or absorbed. The intensity of the transmitted light is reduced as it travels through a polarizing film.

If two polarizing filters are placed in the path of the light and the slits are aligned in parallel, the light will pass through both filters. If the slits of the second filter are perpendicular to the first, no light will pass through the second filter. If the slits of each filter are at an angle to each other, then the component of the light wave parallel to the slits will travel through the filter with reduced intensity. As the angle between the slits of the two filters increases, the light intensity decreases. The second polarizer can determine the orientation of the polarization of light and is therefore referred to as an analyzer (see Figure 7). Polarization can also occur as a result of reflection. Light reflecting off horizontal surfaces, such as water or pavement, is partially polarized in the horizontal direction. Therefore, polarizing sunglasses have polarizing film that is positioned vertically to reduce glare off reflected objects.

Some plastics are composed of materials that are optically active. An optically active material will rotate plane-polarized light. However, the optically active material will rotate different wavelengths of light (colors of light) by varying amounts. This means that each color will be rotated by a slightly different amount. So, when optically active plastic is placed at an angle with respect to the polarized light, the color that transmits through a second polarizer (the analyzer) will depend on the orientation of the analyzer.

Optically active materials and polarized light are often used to test the strength of an object without destroying it. Models of new objects are first made out of clear plastic and then observed under polarized light. Stresses in the plastic cause it to become deformed, altering its optical properties. The result is the display of various colors where the material is stressed due to changes in the refractive index of the material. The rainbow of colors seen in the plasticware pieces visibly illustrates the stresses in the plastic.

Experiment 5: Color

White light is composed of all wavelengths of light from the visible spectrum as well as light wavelengths that are invisible to our eyes (i.e., infrared and ultraviolet). Classically, the visible spectrum has been called “ROY G BIV”: Red, Orange, Yellow, Green, Blue, Indigo and Violet. Actually, there are an infinite number of colors in the visible spectrum range (see Figure 8). A prism refracts, or bends, light. Because a material’s index of refraction is slightly dependent on the wavelength of light, each wavelength bends by a different amount. This leads to the color (wavelength) separation that “changes” white light into what is commonly seen as the rainbow.

A common misconception associated with color is the primary colors. Primary colors are colors that cannot be formed by mixing other colors together. The primary colors of light are red, green and blue. These are often confused with the primary colors of paint (pigments), which are red, yellow and blue. The color observed from the process of mixing paint is the result of color mixing by subtraction (or absorption). Blending colors of light form a color resulting from a process known as color mixing by addition.

Color mixing by subtraction occurs when light illuminating an object is modified by the object before it reflects back to our Eyes (i.e., some color wavelengths are absorbed). The color that is perceived is mostly the complementary color of the wavelength absorbed. Complementary colors are those across from each other on the color wheel (see Figure 8). For example, blue paint pigments reflect blue light, as well as violet and green at lower intensities, and absorb the oranges, reds and yellows. Yellow paint reflects mostly yellow light, and to a lower degree, red, orange and green wavelengths of light. Therefore, when blue paint and yellow paint are mixed and illuminated by white light, the blue pigments absorb red, orange, and yellow wavelengths, and the yellow pigments absorb blue and violet wavelengths. The only color left to be reflected is green, which explains why mixing blue and yellow paint results in the color green. Our eyes see the reflected light from the object and our brain interprets the color of the object as being what humans call “green.” Color mixing by addition takes place when the blended light is unmodified when it illuminates an object and reflects back to our eyes. For example, when two colors of light superimpose on a white sheet of paper both wavelengths (actually all wavelengths) will reflect back to our eyes unaltered. The different reflected wavelengths of light add together and our brain recognizes this “combination” wavelength as a different color. If equal amounts of red and green light wavelengths add together, the combination color appears yellow. When equal amounts of red, green and blue wavelengths of light mix, white light is observed. An interesting property of yellow light and blue light is that when equal amounts of these wavelengths are added together, they produce white light. Yellow and blue wavelengths of light are known as complementary colors of light. Complementary colors are two colors that, when blended together, appear as white light. The perception of the color “yellow” by our brain is the result of either ordinary yellow wavelengths, or the result of the blending of red and green wavelengths of light. Our brain cannot tell the difference. Therefore, when equal amounts of yellow and blue light blend together, our brain interprets this combination the same as it would if equal amounts of red, green and blue light wavelengths were mixed. Magenta and green, and cyan and red are also complementary colors of light. Blending different wavelengths and intensities of light generate an enormous variety of colors.

Newton’s color wheel shows the nature of color mixing by addition and subtraction. White light hits the surface of the spinning disc but reflects back to our eyes in different colors based on the colors of the disc pattern (the other colors are absorbed). When a disc rotates faster than the persistence of vision, the human eye cannot distinguish between the two colors on the disc. The eye “sees” the colors at the same time and therefore the reflected colors are mixed by addition. The disc containing alternating blue- and green-colored sections will appear to be a single different color—cyan. Likewise, a spinning red and blue disc will appear magenta, and a spinning red and green disc will appear yellow.

It is important to recognize that the color of an object depends on the light that illuminates it. An object that is blue under white light will not appear blue under pure red light because blue light is not present to reflect. Most objects reflect more than one color, but the amount that is reflected or absorbed varies. Low-intensity reflected colors can be observed when the dominantly reflected color is removed from the illuminating light source. For example, if a red light illuminates a normally blue object, the object will appear reddish if the object reflects a small amount of red light. This object has always reflected red light, but the red is usually masked by the blue light. If the blue object happens to absorb all of the red light, the object would actually appear black when illuminated by the red light.

In 1894, the toymaker Charles E. Benham introduced his “Artificial Spectrum Top,” which later became known as the Benham’s top or Benham’s disk. When spun, the black and white top appeared to produce colored rings. The appearance of color is still a mystery even after more than 100 years, but it is believed to be at least partially the result of complex nerve responses in the cones of the retina. The cones of the retina allow individuals to see colors. The “pattern-induced flicker colors” that are produced by the Benham’s disk are known as subjective colors because the colors are perceived by our eyes due to the different response times of the cones of the retina. There are three color-sensitive cones on the retina—one for green, one for blue and one for red. Each type of cone has different response and persistence times. For example, the “blue” cones have the slowest response times but they will continue to respond for the longest time after the stimulus has been removed.

When the Benham’s disk spins, alternating flashes of black and white stimulate the cones of the retina. White light has all three primary colors of light—red, green and blue. However, the brain only perceives white light when all the cones respond to the three primary colors equally. When the disk spins, each arc “flashes” at a different rate because each arc has a different amount of white space before and after. Lines that spin “into the black” with the least amount of white space between the black half-circle and the arc appear to be red in color. The middle arcs, with equal white space on each side, appear to be green. The arcs with the most white space between the arc and the black half-circle are blue. When the direction of the spinning disk is reversed, the arc that had the least amount of white space now has the most white space in the direction of the spin. This arc now appears blue. The middle arc still appears green, and the arc that previously appeared blue is now red.

Experiment Overview

Experiment 1: Mirrors and Reflection
Study the properties of reflected light from a flat, shiny surface.

Experiment 2: Refraction
In this experiment, learn about the behavior of light as it travels from one media to another.

Experiment 3: Thin Lenses
We wear eyeglasses to correct our vision, a magnifying glass to see small print and a telescope to see distant objects. What do these devices have in common—they all use lenses. Experiment with various lenses to learn about the properties of different types of lenses and lens configurations.

Experiment 4: Polarization
Experiment with the polarization of light and learn about some of its important industrial uses.

Experiment 5: Color
Why is a red apple red? Does a green filter change white light into green light? What color of light is observed when red and green lights are mixed?

Materials

Safety Precautions

Handle the pins with care. Do not look directly into the flashlight with or without a lens. Do NOT look directly at the sun through the polarizer. Do not look directly at the sun through the prism. Please follow all normal laboratory safety guidelines.

Procedure

Experiment 1: Mirrors and Reflection

1. Obtain the flat mirror, mirror support, protractor sheet, corks and pin.
2. Place the pin in the small end of the cork.
3. Place the mirror in the mirror support stand as shown in Figure 9.
   {13551_Procedure_Figure_9}
4. Place the mirror in the center of the protractor sheet as shown in Figure 9. The mirror should be parallel with the horizontal line on the protractor sheet.
5. Place the second, lone cork at the 40° angle in Quadrant 4 on the protractor sheet (see Figure 10).
   {13551_Procedure_Figure_10}
6. Close one eye and from Quadrant 3, look at the reflection of the cork in the mirror.
7. Line up the image of the cork with the center of the protractor and then place the cork and pin on the backside of the mirror (Quadrants 1 or 2) at the location where the image appears to be. Holding the pin will allow the cork to be more accurately positioned behind the mirror.
8. At what angle and Quadrant was the cork placed behind the mirror? How does the size of the image compare to the real object? Record all observations and data in the Mirrors and Reflection Worksheet.
9. Observe the reflected object in the mirror. Move your head so that the image and the center of the protractor sheet are in line. Note: It may be necessary to close one eye and view the image with your eye near the surface of the tabletop to line up the center point and the image accurately. At what angle and Quadrant on the protractor sheet is your eye located when viewing the image through the center point? Record the angle and Quadrant number in the Mirrors and Reflection Worksheet.
10. Repeat steps 5–9 for two other cork positions.

Experiment 2: Refraction

1. Obtain the semicircular dish, ruler and black permanent marking pen.
2. Measure the length of the flat side of the semicircular dish.
3. Divide the length by 2 to determine the midpoint of the flat side of the dish.
4. Measure from the edge to the midpoint on the flat side of the semicircular dish and make a thin vertical line with the marking pen.
5. Fill the dish ¾-full with water.
6. Place the dish on the protractor sheet so that the flat side is in line with the zero line, the midpoint line on the dish is centered over the center point on the protractor sheet and the curved portion of the dish is in Quadrants 1 and 2 (see Figure 11).
   {13551_Procedure_Figure_11}
7. Place a pin in the small end of the cork.
8. Place the cork and pin at the 50° mark in Quadrant 4 on the protractor.
9. Look through the water-filled dish from the 50° mark in Quadrant 1 of the protractor sheet, so that your line of sight goes through the center point on the protractor sheet. It may be necessary have your eye level at the same height as the tabletop. Describe what you see in the Refraction Worksheet. Optional: Draw a picture of what you see.
10. Move your head and eyes along the curved part of the dish until the cork and pin image “in the water” line up with the vertical line on the flat side of the dish.
11. With the pin and vertical line aligned, line up the lone pin on the semicircular dish side of the protractor sheet to mark the line-of-sight angle. For best positioning, look through the dish so that your eye is at tabletop level.
12. Record this angle in the Refraction Worksheet.
13. Repeat steps 8–12 at two different cork and pin positions (angles).
14. Repeat steps 8–13, but this time place the cork and pin on the same side as the semi-circular dish (Quadrant 1) and look through the water from the flat side (Quadrant 4). Line up the cork and pin image “in the water” with the vertical line in the same manner. Record your observations and data in the Refraction Worksheet.

Experiment 3: Thin Lenses

One-Lens System

1. Obtain the meter stick, lens support, pin, cork, target card, lenses and a flashlight.
2. Place the pin in small end of the cork.
3. Lay the meter stick flat on the tabletop so that the 100-cm side is near the edge making it possible to get your head low enough to look directly down the length of the meter stick.
4. Place the cork and pin at the 10-cm mark on the meter stick.
5. Place the 15-cm focal length, convex lens in the lens support (see Figure 12).
   {13551_Procedure_Figure_12}
6. Place the 15-cm lens at the 60-cm mark on the meter stick.
7. Shine the flashlight from the zero mark on the meter stick, toward the lens. The pin will cast a shadow on the lens.
8. Hold the flashlight steady, and hold the optics target card vertically on the backside of the lens (at approximately the 70 cm mark).
9. Move the target card along the meter stick until a clear, focused image of the pin’s shadow is cast onto the card. Move the card until the image is as large and as clear as possible. Is the image of the pin upright or upside-down (inverted)? Determine the position of the target card on the meter stick. Record all observations and the position of the target card in the Thin Lenses Worksheet.
10. Move the lens 10–20 cm closer to the object and repeat steps 4–9. Record the positions of the object, the lens and the focused image in the Thin Lenses Worksheet.
11. Repeat steps 4–10 using the 5-cm focal length convex lens.
12. Repeat steps 4–9 with the 5-cm focal length concave lens.

Two-Lens System

1. Place the cork and pin at the 10-cm mark on the meter stick.
2. Place the 15-cm focal length, convex lens in the lens holder.
3. Place the 15-cm lens at the 70-cm mark on the meter stick.
4. Hold the 5-cm focal length convex lens on the backside of the 15-cm lens (at approximately 85 cm).
5. Holding your head at the same level as the 15-cm lens, look through the 5-cm lens toward the 15-cm lens. Initially, hold the 5-cm convex lens approximately 5 cm in front of your eye. Caution: Make sure the flashlight is turned off before looking through the lens system.
6. Move the lens forward or back along the meter stick until the image of the pin and cork come into focus. It may require a steady hand and practice when positioning the 5-cm convex lens properly in order to view a clear image of the pin and cork.
7. Once the image is clear when your eye is approximately 5 cm from the lens, close one eye, move your head closer to the 5-cm lens, and look through it as a simple magnifier.
8. Move your head and the lens back and forth slightly until the pin and cork are as large as possible and in focus.
9. When this is accomplished, place the 5-cm convex lens on the meter stick to mark its location.
10. Record the position of the 5-cm convex lens in the Thin Lenses Worksheet. Is the image upright or upsidedown? Record all observations in the worksheet.
11. Repeat steps 4–10 using the 5-cm focal length concave lens.

Experiment 4: Polarization

1. Obtain two polarizing film squares and look through each film at a light source (e.g., a light in the classroom, from a window). Caution: Do not look directly at the sun through the polarizer. Record the appearance of the light transmitted through the polarizing film in the Polarization Worksheet.
2. Stack the two polarizing films, one on top of the other, and look through the two-layer film at the same light source. Record the initial appearance of the light transmitted through both films.
3. Rotate one of the films in a circle while keeping the other film in its original orientation. Record the appearance of the transmitted light as the film rotates. What are the specific orientations of the films in which all the light is blocked and all the light is transmitted? Record all observations and data in the Polarization Worksheet.
4. Obtain the clear plastic “silverware” and the plastic bag.
5. Cross the two polarizing films so no light is transmitted. Then place the plastic bag between the two crossed films. Keep the polarizing films in the same orientation and rotate the bag between them. How does the transmitted light appear when the plastic bag is at a diagonal compared to the axis of the polarizing film? Record all observations in the worksheet.
6. Obtain the plasticware and hold it on top of one of the polarizers. Hold the polarizer and plasticware up to the light (with the polarizer between the light source and the plasticware) and then look at them through the other polarizer (see Figure 13}. Rotate the lone polarizer (the “analyzer”) and record all observations in the worksheet.
   {13551_Procedure_Figure_13}
7. Repeat step 6 for the other piece of plasticware.

Experiment 5: Color

Prism—Teacher Demonstration 

1. Record observations in the Color Worksheet.

Color Wheels

1. Obtain the three color wheels and a pin.
2. (If necessary) Poke a hole in the center of each color wheel using the pin. Make sure the hole is not bigger than the head of the pin.
3. Insert the pin into the hole in the largest color wheel (blue and green) so the color side is up and the pinpoint is down.
4. Hold the pin near the point and quickly spin the wheel on the pin. Spin the wheel as quickly as possible. The head of the pin should prevent the wheel from flying off the pin.
5. Observe the color pattern produced by the color wheel. Record the observed color(s) in the Color Worksheet.
6. Remove the largest color wheel and add the medium-size color wheel (red and green).
7. Repeat steps 3 and 4 for the medium-size color wheel.
8. Remove the medium-size color wheel and add the smallest color wheel (blue and red) to the pin.
9. Repeat steps 3 and 4 for the smallest color wheel.

Color Filter Paddles

1. Obtain the color filter paddles.
2. Cross the red and yellow color paddles. What color is observed? Record this information in the Color Worksheet.
3. Repeat step 2 with the blue and yellow paddles, and then the red and blue paddles. Record the observed colors in the worksheet.
4. Now, cross the red, yellow and green filters. Record the observed color in the worksheet.
5. Cross the red, yellow and blue filters. Record the observed color in the worksheet.
6. Cross other filter paddles to view the new color. Record the color of the filter paddles that were crossed and the observed color in the worksheet.

Benham’s Disk

1. Obtain a blank, laminated Benham’s disk, a pin and a black dry erase marker.
2. Use the black dry erase marker to draw arcs on the white portion of the disk similar to those shown in Figure 14.
   {13551_Procedure_Figure_14}
3. (If necessary) Poke a hole in the center of the Benham’s disk using the pin. Make sure the hole is not bigger than the head of the pin.
4. Insert the pin into the hole in the Benham’s disk so the black and white side is up and the pinpoint is down.
5. Record the initial color(s) on the Benham’s disk in the Color Worksheet.
6. Hold the pin by the point and hold the disk parallel to the ground with the black and white side up.
7. Carefully, yet quickly, rotate the disk clockwise on the pin by continually pushing the outside edge with your free hand. Continue rotating the disk and observe the pattern created by the spinning disk. Are any colors observed? When the disk stops rotating, are there any changes from the original appearance? Record all observations on the Color Worksheet.
8. Rotate the disk counterclockwise on the pin. Continue to rotate the disk and observe the pattern created by the spinning disk. Are any colors observed? Have the locations of the colors changed? Record all observations on the Color Worksheet.
9. Use a paper towel to remove the black marks from the Benham’s disk.
10. Review your observations to develop a hypothesis explaining the observed changes. Enter this hypothesis in the Color Worksheet.

Student Worksheet PDF

13551_Student1.pdf