Materials Included In Kit

Additional Materials Required

Disposal

The materials should be saved and stored for future use.

Lab Hints

• Enough materials are provided to build eight microscopes for eight groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The Prelab Questions may be assigned as homework or be used during a pre-lab discussion.
• The Prelab Microscope Construction can be performed by the students or by the teacher to save classroom time.
• It is often best to look into the microscope with both eyes open to eliminate squinting and eye strain. Students should look through the microscope eyepiece with only one eye and cover the other open eye with their free hand.
• Sometimes it is easier for students to look through the microscope with their head (and eye) a few centimeters from the eyepiece lens when trying to find the “e.” The field of view will be larger and this will limit eye strain. However, once the students see the image, they should bring their eye near the eyepiece lens so that they use it as a simple magnifier to produce the largest image. Note: Students may believe the image is larger when they look through the microscope from a distance because the field of view is larger (the image fills up the entire lens). However, the image is largest when they place their eye near the lens and use the eyepiece lens as a simple magnifier. The field of view may be smaller, but the image is larger.
• Students may want to hold the microscope vertically like a “real” microscope to view the object. The microscope will work fine in this orientation, but this technique typically does not produce the best measurement data because it is difficult to hold the microscope steady (with the image of the “e” in focus) as another lab partner measures the distance between the bottom edge of the microscope and the tabletop (the index card). Students will need to hold the microscope directly over the “e” and move the microscope up slowly and steadily, making sure the “e” stays in the field of view until it becomes clear. Students must also securely hold both parts of the microscope so its size does not change during the focusing process.
• The microscope may also be mounted vertically using a ring stand and clamps. Leave about 10 cm between the objective and the tabletop to allow for the movement of the index card during the focusing process.
• Wipe the eyepiece lenses (5-cm dia.) with rubbing alcohol (70% isopropyl alcohol) to disinfect them between classes, if necessary.
• White glue can be used to permanently secure the lens units inside the tubes.
• The amount of tape necessary to keep the lens units secure inside the tubes will depend on the thickness of the tape. To limit the amount of tearing that can occur with paper tubes when the lens units are taken apart, masking tape is recommended. Also, use as little tape as possible to secure the rings if they are going to be taken apart frequently.
• To better estimate the true focal length of the convex lenses, hold the lens in sunlight to form a very sharp point of light on the ground. Make sure that the light does not converge onto any combustible material. Measure, to the nearest mm, the distance between the lens and this sharp point of light. This is the true focal length of the lens. This measured focal length can than be used to determine the magnification of the microscope.
• Over time, the paper tubes may shrink as moisture evaporates from the paper. Once the paper is completely dry, the tubes’ sizes will no longer change. However, the final “dry” size may affect how well the pieces fit inside one another. If a piece is too loose inside a tube, use masking tape to tape around the outside of the tube to add to the diameter of a tube. If a ring’s diameter is too large to fit inside a tube, a small section of the ring can be clipped with scissors. The ends of the cut sections can then be squeezed together so that the ring fits inside the tube. The rigidity of the ring will cause it to expand inside the tube and therefore create a snug fit. Clip the ring starting with about a ¼" section. If the ring is still too large, clip additional segments until the ring fits inside the tube.
  {12584_Hints_Figure_6}

Teacher Tips

• 100X objective lenses are typically oil immersion lenses, in which a drop of oil is placed between the objective lens and the cover slip. The refractive index of oil and the cover slip are closely matched so the light transmitted from the viewed specimen has minimal refraction. This lens is so powerful that a small amount of refraction will cause the image to be blurry.
• Students may notice that the images formed by the microscopes will be distorted. This distortion is known as aberration. For thin, theoretical lenses, aberration is neglected. For thick, real-life lenses, aberration is natural and correcting or minimizing it is important for producing the best images. Modern microscopes use complicated optics to eliminate distortion and create sharp, finely tuned images. Aberration is not caused by defects in the lenses, but is due to the laws of refraction and reflection on spherical surfaces. That is, since lenses are not uniformly thick (thicker in the middle than at the edges for a convex lens) the lens will not bend light uniformly from the center of the lens to the edge of the lens. Therefore, the focused rays do not all meet at the focus of the lens resulting in a distorted or blurry image.
• Students should refer to their physical science or physics textbook for more information and examples using the thin-lens equation.
• The Build a Telescope Kits (Catalog Numbers AP6266 and AP6297) are excellent complements to the Build a Microscope Kit, and are available through Flinn Scientific.

Answers to Prelab Questions

1. Define linear magnification and angular magnification. Why must angular magnification be used for the eyepiece lens?

   Linear magnification is the ratio of the image size to the object size. Angular magnification is defined in the Background section as the ratio of the viewing angle needed to see the entire virtual image (while looking through the lens) to the viewing angle needed to see the object without the lens. Angular magnification is used for the eyepiece lens because the final image is a virtual image, located on the light-entering side of the lens, meaning no physical dimensions can be measured.

2. An object is placed 6 cm in front of a 5-cm focal length lens. At what position does the image form? Calculate the linear magnification created by this objective lens. Is the image real or virtual? Is the image upright or inverted?

   Using Equation 1: 1/(5 cm) = 1/(6 cm) + 1/i → 1/i = 1/(5 cm) – 1/(1/6 cm) = 0.0333/cm
   i = 30 cm
   
   Using Equation 2: M = –i/o = –30 cm/6 cm = –5
   The image is real and inverted.

3. The separation distance between a 5-cm focal length objective lens and a 10-cm focal length eyepiece lens is 39 cm. Using your answer for the image distance in Prelab Question 2, where is the image formed by the objective lens located in relation to the front focal point of the eyepiece lens? What is the distance between the image and the front focal point?

   The image formed by the objective lens is located 30 cm from the objective lens. Therefore, the image is 9 cm from the eyepiece lens. The image forms between the front focal length of the eyepiece lens and the actual eyepiece lens. The distance between the image and the front focal length is 1 cm.

4. Using the image location in relation to the eyepiece lens calculated in Prelab Question 3, calculate the location of the image produced by the eyepiece lens. Is the image real or virtual? Is the image upright or inverted?

   Using Equation 1: 1/(10 cm) = 1/(9 cm) + 1/i → 1/i = 1/(10 cm) – 1/(9 cm) = –0.01111/cm
   i = –90 cm
   The image is virtual and inverted.

Sample Data

Answers to Questions

1. How does the image of the object change as the length of the microscope increases?

   The clear, focused image increased in size as the microscope expanded.

2. The centers of the objective and eyepieces lenses are both approximately 1.5 cm from the ends of the microscope tubes. Calculate the true separation distance between the objective and eyepiece lenses for each microscope length.

   Microscope length	Lens separation
   30 cm	27 cm

3. Find each microscope length, calculate the average value for the Distance Between the Object and the Objective End of the Microscope. Then, add 1.5 cm to each average distance to calculate the total distance between the object and the center of the objective lens.

   Sample calculations
   (5.4 cm + 5.3 cm)/2 = 5.35 cm → 5.4 cm + 1.5 cm = 6.9 cm

   Microscope length	Object distance (o)
   30 cm	6.9 cm
   35 cm	6.5 cm
   40 cm	6.1 cm

4. Use Equation 1 from the Background to calculate the position of the real image (image distance) formed by the objective lens, for each microscope length. Note: The focal length of the objective lens is 5 cm.

   Sample calculation
   1/(5.0 cm) = 1/(6.9 cm) + 1/i → i = 18.2 cm → 18 cm

   Microscope length	Image position
   30 cm	18 cm
   35 cm	22 cm
   40 cm	28 cm

5. Where does the position of the real image fall in relation to the front focal point of the eyepiece lens?

   The position of the image is just inside the 10-cm front focal length of the eyepiece lens. In other words, the image forms about one centimeter “behind” the eyepiece’s front focal point, between the front focal point and the actual eyepiece lens.

6. Use Equations 2, 3 and 4 to calculate the magnification of each microscope. (Assume the real image produced by the objective lens is located at the front focal point of the eyepiece.)

   Sample calculations

   {12584_Answers_Equation_5}

   {12584_Answers_Equation_6}

   Microscope length	Magnification
   30 cm	6.5
   35 cm	8.5
   40 cm	11.5

   M_microscope = 2.6 × 2.5 = 6.5

7. Why does the microscope’s magnification increase as the length of the microscope tube expands?

   A longer microscope tube allows the objective lens to be placed close to the object, and increases the distance the image forms from the objective lens. The linear magnification ratio (i⁄o) increases with increasing i and decreasing o. Therefore, the linear magnification increases. The angular magnification of the eyepiece remains the same because it is assumed the image from the objective is located at the front focal point of the eyepiece lens.

Advanced Post-Lab Questions

8. Draw (to scale) the positions of the object, intermediate images and final image on the retina of the eye for the 30-cm long microscope used in this experiment. Assume the focal length of the eye’s lens is 1.7 cm, and is centered at approximately the same location as the eyepiece lens. Scale: Each mark represent 3 cm. (Optional) On a separate sheet of paper, draw the ray diagram for the 30-cm long microscope.
   {12584_Answers_Figure_7}
   Note: An intermediate virtual image forms 90 cm to the left (on the incident side) of the eyepiece lens and is not included in the diagram due to space restrictions. This virtual image is the “object” the eye focuses on to form a real image on the retina. The final image is real and upright. However, our brain will perceive this “upright” image as being upside-down, or inverted. The convex lens of the eye inverts all the images so the real images are upside-down on the retina. The brain corrects for this so we “see” right-side-up. If students decide to draw a ray diagram, the results should be similar to those above. Lines must be straight and parallel, and measurement scales must be accurate in order to draw a properly represented ray diagram.

References

Tipler, Paul A.; Physics for Scientists and Engineers, 3rd Ed., Vol. 2; Worth Publishers: New York, 1990, pp 1041–1058.

en.wikipedia.org/wiki/Magnification; en.wikipedia.org/wiki/Optical_microscope (accessed 2-1-2007)

Introduction

Microscopes allow scientists to observe samples in detail far exceeding the limits of the human eye. Many different varieties of microscopes exist, ranging from simple magnifiers to complex electron microscopes with the capacity to view objects at up to two million times their original size!

Concepts

• Microscopes
• Thin-lens equation
• Convex lenses
• Focal length

Background

Throughout recent history, microscopes have proven to be a vital instrument in scientific advancements. Although the inventor of the first microscope is not entirely clear, credit is usually given to Zacharias Janssen (1580–1638) and his father Hans for the crude microscope they created around 1595. Years later, the experimental work of Galileo Galilei (1564–1642), Robert Hooke (1635–1703), and Anton van Leeuwenhoek (1632–1723) vastly improved the power of the compound microscope. Hooke performed one of the first microscopy experiments to discover why cork was so light and buoyant. Using his microscope, Hooke saw that cork, in fact, consisted of small chambers filled with air, which he called “cells.”

The most common laboratory microscope, the compound microscope, has glass lenses in the ocular, or eyepiece, and in each objective. The objective and eyepiece lenses of a microscope are both convex, or converging, lenses. Convex lenses bend light to pass through a point on the light-exiting (transmission) side of the lens. The light is said to “focus” through this point, so it is known as the focal point. The focal point of any lens is the point at which a beam of light parallel to the principal axis of the lens converges (see Figure 1).

The objective lens focuses incoming light from an object located near the focal point through the back focal point of the lens to form a magnified real image on the transmission side of the lens (see Figure 2). A real image is an image that can be formed on a screen and therefore can be seen by the naked eye. The eyepiece lens in combination with the lens of the eye, acts as a simple magnifier to further magnify the enlarged real image produced by the objective lens, and to produce a virtual image that is easily focused on by the eye. A virtual image is an image that forms on the incident side (incoming-light side) of the lens (see Figure 3). Unlike a real image, a virtual image can only be seen when looking directly through the lens. The virtual image will not form on a screen. Therefore, when looking through a microscope, the eye sees this final virtual image as an apparent enlarged object “inside” the microscope. The convex lens of the eye focuses the incoming light from this “object” to form a large real image on the retina. The result—the original object appears much larger than it did with the unaided eye.

The position of an image formed by any thin lens in relation to the object’s position, or distance from the lens, can be determined using the thin-lens equation shown in Equation 1, or by drawing ray diagrams as in Figures 1–3. This equation can also be used in a stepwise fashion for multiple lens systems, such as that for a microscope, to determine the location of the final image.

f = focal length
i = image distance from lens
o = object distance from lens

The thin-lens equation uses the convention that convex lenses have positive focal lengths and concave lenses have negative focal lengths. The focal length is the distance between the center of the lens and the focal point of the lens. Another convention is that light travels from left to right. Therefore, objects to the left (on the incident side) of the focusing lens and images formed to the right (on the transmission side) of the focusing lens have positive distances and are real. Objects to the right (transmission side) and images formed to the left (incident side) of the focusing lens have negative distances and are virtual.

The linear magnification of a thin lens can be determined whenever a real image forms, because its location and size can be measured. Linear magnification is given by Equation 2. A negative magnification means the image is inverted and real.

M_l = linear magnification
h_i = image height
h_o = object height

Simple magnifiers allow an object to be placed closer to the eye than the near point of the eye and allow the eye to focus on an enlarged image without eye strain. The near point of the eye is the closest distance an object can be placed in front of the eye so that the eye’s lens can still clearly focus the image on the retina. Any object positioned closer than the near point will be blurry. For a “normal” eye, the near point is 25 centimeters. When an object is positioned at the front focal point of a simple magnifier, whose focal length is shorter than the near point distance, holding the magnifier very close to the eye allows the eye to focus on the enlarged virtual image produced by the magnifying lens. The effective angular magnification of a simple magnifier when the lens is held close to the eye and the viewed object is at the front focal point of the magnifier is given by Equation 3. Angular magnification is the ratio of the viewing angle needed to see the entire virtual image (while looking through the lens) to the viewing angle needed to see the object without the lens (at the near point). Angular magnification is calculated whenever the final image is virtual because it is not possible to determine the image’s true size with a physical measurement.

M_ep = angular magnification of eyepiece
25 cm = near point of the eye
f_e = focal length of magnifier (eyepiece)

The magnification of the eyepiece in a typical compound microscope is 10X, and the most commonly used objective lenses are 4X, 10X, 40X and 100X magnifications. The total magnification of a microscope is found by multiplying the magnification of the eyepiece (M_ep) by the magnification of the objective (M_o) in use (Equation 4). The magnification of the objective is determined by its linear magnification, M_l (Equation 2).

Experiment Overview

Build a replica of one of the first microscopes and determine how the lens combination magnifies very small objects.

Materials

Prelab Questions

1. Define linear magnification and angular magnification. Why must angular magnification be used for the eyepiece lens? 
2. An object is placed 6 cm in front of a 5-cm focal length lens. At what position does the image form? Calculate the linear magnification created by this objective lens. Is the image real or virtual? Is the image upright or inverted?
3. The separation distance between a 5-cm focal length objective lens and a 10-cm focal length eyepiece lens is 30 cm. Using your answer for the image distance in Prelab Question 2, where is the image formed by the objective lens located in relation to the front focal point of the eyepiece lens? What is the distance between the image and the front focal point?
4. Using the image location in relation to the eyepiece lens calculated in Prelab Question 3, calculate the location of the image produced by the eyepiece lens. Is the image real or virtual? Is the image upright or inverted?

Safety Precautions

The materials in this lab are considered safe. Warn students not look at the sun or bright light sources while using the lenses. Students should follow all laboratory safety guidelines.

Procedure

Prelab Microscope Construction (Refer to Figure 4 for a diagram of the microscope pieces.)

1. To build the eyepiece lens unit, sandwich the 50-mm diameter convex lens (O) between the two 1¾" i.d. x ½" wide paper rings (A). Apply a small amount of masking tape around the seam to secure the rings and the lens.
2. To build the objective lens unit, sandwich the 38-mm diameter convex lens (E) between two 1⅜" i.d. x ½" wide paper rings (B) inside a 1½" i.d. x 1⅛" long short tube (C). The lens should be centered inside the tube and secure. A small amount of tape may be necessary to hold the rings inside the 1⅛” long tube.
3. Slide the eyepiece lens unit inside the 2" i.d. x 8" long tube until the ends of the tube and the eyepiece lens unit’s rings are flush. If the lens unit fits loosely, wrap additional tape around the rings until the eyepiece lens unit will slide inside the tube and remain tight and secure at the end. Alternately, “double-backing” a long piece of tape to form a ring of tape with the sticky-side out can also be used to secure the eyepiece lens unit inside the tube.
4. Slide the objective lens unit into one end of the 1¾" i.d. x 8" long microscope tube until the end of the unit is flush with the end of the tube. If the objective lens unit fits loosely, wrap tape around the unit until it fits tightly inside the end of the microscope tube.
5. To complete the microscope: Insert the 1¾" i.d. x 8" long microscope tube, with the eyepiece lens unit, into the 2" i.d. x 8" long microscope tube so that the lenses are on opposite ends.
   {12584_Preparation_Figure_4_Microscope construction}

Experiment

1. Near one of the corners on an index card, make a small letter “e” using a colored pen. Colored ink produces an easy-to-recognize image in the microscope. As a reference, draw a copy of the “e” approximately the same size in the upper righthand corner of the “observations box” on the Build a Microscope Worksheet.
2. Place a meter stick on a tabletop so that its zero-mark is at the edge of the table top.
3. Place the completed microscope against the meter stick so that it does not roll, and position the eyepiece end of the microscope at the zero-mark on the meter stick (see Figure 5).
   {12584_Procedure_Figure_5_Top view}
4. Adjust the total length of the microscope to 30 cm. Make sure the eyepiece lens and objective lens units are flush with the ends of the tube.
5. Place the index card with the letter “e” facing the objective lens of the microscope. Note: Hold the index card vertically and perpendicularly to the meter stick. The position of the index card will be used to measure the separation distance between the object (“e”) and the end of the microscope.
6. Place your head right up to the eyepiece of the microscope and look through the eyepiece lens. Slowly adjust the location of the index card to bring the “e” into focus. Do not change the length of the microscope. Note: It will be necessary to move the card toward or away from the microscope and to move it up and down or side to side until the “e” is visible, large and focused. This will require practice for the first couple of measurements. Use slow movements because the magnified image of the “e” will “move” very quickly in the field of view of the microscope.
7. Once the image of the “e” is large and in focus (showing the most detail), line up the index card so that it is vertical and perpendicular to the meter stick, making sure the “e” remains in focus (see Figure 5).
8. Measure the distance between the index card and the front edge of the microscope (the objective side of the microscope) to the nearest 0.1 cm. The index card and the front end of the microscope will line up with markings on the meter stick, so the two positions can be subtracted to determine the distance between them. Record this distance in the data table on the Build a Microscope Worksheet.
9. Observe the appearance (e.g., size, orientation, distortion) of the image and record all observations on the Build a Microscope Worksheet. Sketch the image to show its relative size compared to the original “e” on the index card.
10. Each lab partner should switch roles and repeat steps 5–9. Record the second measured distance between the microscope and the index card, and any new observations in the data table under trial 2.
11. Adjust the length of the microscope to 35 cm.
12. Repeat steps 5–10. Record the measured distances and observations in the data table. How does this image compare to the previous image? Sketch the new image to show its relative size compared to the first image.
13. Adjust the length of the microscope to 40 cm.
14. Repeat steps 5–10. Record the measured distances and observations in the data table. How does this image compare to the previous two images? Sketch the new image to show its relative size to the first two images.
15. Consult your instructor for appropriate storage procedures.

Student Worksheet PDF

12584_Student.pdf