Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Insert the batteries into the laser pointers.
2. Construct the measurement frames by cutting each index card into 2½" x 3" pieces. Using scissors or a utility knife, cut a center rectangle out of each index card half, leaving a 1.5-cm border around each side (see Figure 9). One frame is needed per student group.
   {12114_Preparation_Figure_9}

Safety Precautions

Remind students to not aim the laser pointer directly into anyone’s eyes and not to look directly into the beam. The low-power, coherent light can cause damage to the sensitive retina and may lead to permanent eye damage. Students should not aim the laser at any reflective surfaces, such as mirrors or highly polished metal. Prevent stray laser light from projecting beyond the classroom to eliminate any unintentional exposure to the laser light. When refracting the laser light, it is best to do this on a low work surface to keep the refracted laser light below “normal” eye level. For people with sensitive eyes it is recommended that dark, IR- protective safety glasses be worn. Follow all other normal laboratory safety guidelines.

Disposal

Remove batteries from laser pens for long-term storage. Measurement frames with the fishing line, copper wire and hair may be disposed of in the regular trash.

Lab Hints

• Enough materials are provided in this kit for 6 groups of students. This laboratory activity can reasonably be completed in one 45- to 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• The use of lasers in the classroom has significant educational value, and the safe use of lasers in the classroom is possible. Please remind students how to safely use a laser.
• Additional lasers are available from Flinn Scientific, Catalog No. AP8934. Laser pointers may also be purchased at some department stores or pet supply stores. Be sure to purchase lasers that are clearly labeled with the class, power and wavelength.
• The longer the distance, L, the more spread out the diffraction pattern, but the bands will also be less distinct. Distances from 1.5 to 3 meters are recommended for ease of measurement between bands. The distance between bands will be more difficult to measure at distances less than 1 meter.
• The diffraction pattern is more distinct in a dimly lit environment. Turning off classroom overhead lights should be sufficient for viewing the diffraction pattern if other sources of light enter the room. However, a completely dark classroom is not recommended for safety reasons.
• If sufficient wall space is not available for all student groups, a large book or notebook may be used instead. Students should place the setup on the floor, counter or lab table, stand a book up 1.5 to 3 meters from the object being measured and tape a piece of white paper to the book. The cover of the book should be parallel to the measurement frame.
• The laser beam wavelength may vary on the order of ±30 nm from trial to trial because the wavelength depends on the condition of the transistor. Heat affects the transistor properties and therefore the wavelength of the light. A laser that has been used continuously for several minutes may produce a light with a wavelength that is slightly different compared to when it was just turned on. Remind students to only turn on the laser when their setup is ready and to release the power button as soon as they have placed tape between two dark spots on the white paper.

Teacher Tips

• This activity may be used to develop the concepts of measurement, energy and the electromagnetic spectrum or properties of light. This experiment would also be a good supplementary activity to a microscope unit in life science.
• For more information on the laser principle, request Laser Theory, Publication No. 10427.
• If time permits, students may measure the width of a hair from each lab partner and compare results. Students may also bring in other samples to measure (e.g., sewing thread, pet hair, different fishing line).
• Students may continue to explore the properties of laser light with the Flinn Laser Pointer Education Kit, Catalog No. AP4507.

Answers to Prelab Questions

1. Complete the following “If/then” hypothesis to explain how the width of the object being measured will influence the distance between the bands of the diffraction pattern.

   “If an object being measured with laser light is smaller than a previously measured object, then the distance from one band to the next will increase because the width of the object is inversely proportional to the distance between the bands.”

2. The wavelength of visible light is usually measured in nanometers and the width of human hair is usually measured in micrometers (1 μm = 1 x 10^–6 m). (a) How many nanometers are in a micrometer? (b) The light from a green laser pointer has a wavelength of 532 nm. Convert this wavelength to micrometers. Show your work.
   1. 1 μm = 1000 nm
   2. 532 nm x 1 μm/1000 nm = 0.532 μm
3. All the dimensions of Equation 1 from the Background section will be millimeters. (a) If 1 mm = 1000 μm, what is the wavelength from Question 2(b) in millimeters? (b) Suppose the distance, L, is 1.4 meters. How would that be expressed in millimeters?
   1. 0.53 μm x 1 mm/1000 μm = 0.00053 mm
   2. 1.4 m x 1000 mm/1 m = 1400 mm
4. What safety precautions must be taken when using a laser pointer?

   Do not aim the laser pointer directly into anyone’s eyes and never look directly into the laser beam. Do not aim the laser at any reflective surfaces. Prevent stray laser light from projecting beyond the classroom to eliminate any unintentional exposure to the laser light. When refracting the laser light, it is best to do this on a low work surface to keep the refracted laser light below “normal” eye level.

Sample Data

Post-Lab Calculations

Answers to Questions

5. List the measured objects from the data table in order from smallest width to largest. How did the diffraction pattern change from one object to the next?

   The human hair had the smallest width, followed by the copper wire and then the fishing line. The smaller the object, the greater the distance between the bands of the diffraction pattern.

6. The diameter of human hair varies, but is usually in the range of 20–180 μm. Did the experimental value obtained fall within this range? Compare your results with other groups. Does there seem to be a relationship between hair color and width? Explain.

   The hair measured 110 μm, within the accepted range. Lighter-colored hair appears to be thinner than darker hair.

7. The diameter of a 30-gauge copper wire is 0.255 mm. (a) How does the measured width of the copper wire compare to the accepted width? (b) Use Equation 2 to calculate the percent error between the measured and accepted values for the width of the wire. (c) What are some possible sources of error in this experiment?
   1. The measured value for the copper wire was very close to the accepted value.
   2. (0.26 mm – 0.255 mm)/0.255 mm x 100% = 2.0% error
   3. Possible sources of error include difficulty in marking the exact center of the dark bands with tape, variations in the wavelength of the laser light and precision of measuring instruments (meter stick and ruler).

References

PhysicsQuest. Physics Central. http://www.physicscentral.com (accessed April 2013).

Toombes, G. Diffraction. Cornell Center for Materials Research [Online] March 2003. http://www.ccmr.cornell.edu/education/ (accessed April 2013).

Introduction

A variety of methods are available for measuring objects and using the appropriate instrument is important. For example, a ruler may be used to determine the thickness of a book while a meter stick would be more reasonable for measuring the height of a table. What if the object is less than a millimeter wide? Discover how light can be used to measure the dimensions of very small objects, such as the width of a wire or a human hair.

Concepts

• Measurement
• Diffraction
• Interference
• Wavelength

Background

Visible light, like all energy of the electromagnetic spectrum, travels in waves with crests and troughs. The height of the crest is the amplitude and the distance from one crest to the next is the wavelength (see Figure 1).

Each color of the visible spectrum has its own wavelength—measured in nanometers (1 nm = 10^–9 m)—ranging from 400 nm for violet light to 700 nm for red light (see Figure 2). When light strikes the edge of an object, the light bends and spreads out, much like water waves fan out when they strike a barrier. This bending of light is called diffraction. When an object is very small, the light waves bend around both sides of the object and overlap, creating an interference pattern. If the crests of two waves overlap, constructive interference results with the wave amplitude becoming greater, increasing the brightness of the light. If a crest of one wave meets a trough from another wave, the waves cancel out. This is known as destructive interference (see Figure 3). Unlike white light with a range of wavelengths, laser light is monochromatic light—light of one color—and is composed of a single wavelength. When the light of a single wavelength bends around a small object, a distinctive diffraction pattern of light and dark bands is observed. The light bands are a result of constructive interference and the dark bands are a result of destructive interference (see Figure 4). The distance between the bands is inversely proportional to the width of the object. When the diffraction pattern is projected onto a screen, the distance from one dark band to the next can be measured. By using Equation 1, the width of a very thin object can be calculated. where

d is the width of the object
λ is the wavelength of light
L is the distance between the object and the screen
Δy is the distance from the center of one dark band to the center of the next dark band

Experiment Overview

The purpose of this experiment is to use a laser pointer to measure the width of three micrometer-size objects—a fishing line, copper wire and human hair—by analyzing the diffraction pattern produced by each object.

Materials

Prelab Questions

1. Complete the following “If/then” hypothesis to explain how the width of the object being measured will influence the distance between the bands of the diffraction pattern.

   “If the width of an object being measured with laser light is less than a previously measured object, then the distance from one band to the next will (increase/decrease) because ___________________________________________________.”

2. The wavelength of visible light is usually measured in nanometers and the width of human hair is usually measured in micrometers (1 μm = 1 x 10^–6 m). (a) How many nanometers are in a micrometer? (b) The light from a green laser pointer has a wavelength of 532 nm. Convert this wavelength to micrometers. Show your work.
3. All the dimensions of Equation 1 from the Background section will be millimeters. (a) If 1 mm = 1000 μm, what is the wavelength from Question 2(b) above in millimeters? (b) Suppose the distance, L, is 1.4 meters. How would that be expressed in millimeters?
4. What safety precautions must be taken when using a laser pointer?

Safety Precautions

Do not aim the laser pointer directly into anyone’s eyes and never look directly into the laser beam. The low-power, coherent light can cause damage to the sensitive retina and may lead to permanent eye damage. Do not aim the laser at any reflective surfaces, such as mirrors or highly polished metal. Prevent stray laser light from projecting beyond the classroom to eliminate any unintentional exposure to the laser light. When refracting the laser light, it is best to do this on a low work surface to keep the refracted laser light below “normal” eye level. For people with sensitive eyes it is recommended that dark, IR-protective safety glasses be worn. Follow all other normal laboratory safety guidelines.

Procedure

1. Obtain a measurement frame (modified index card) and a 6-cm piece of fishing line.
2. Orient the frame so the longer sides are at the top and bottom. Stretch the fishing line vertically across the center of the frame opening.
3. Tape the fishing line to the top and bottom of the frame, making sure the line is vertical and taut (see Figure 5).
   {12114_Procedure_Figure_5}
4. Obtain a 6-cm piece of 30-gauge copper wire and tape the wire to the frame to the left of the fishing line, leaving about a centimeter of space between them.
5. Obtain a sample of human hair (one person in the group should carefully pull or cut one strand from his or her own head) and tape the hair to the frame to the right of the fishing line (see Figure 5). Cut off any excess length from the hair sample.
6. Attach a small binder clip to one bottom corner of the frame as shown in Figure 5 so the frame will stand up.
7. Obtain a laser pointer. Note: Make sure you have read the safety precautions section regarding the use of lasers. Do not press the power button to turn on the laser until the set-up is complete. Leaving the light on too long will affect its wavelength, which in turn will affect the results.
8. Use the medium binder clip as a stand for the laser by placing the laser pointer inside the medium clip with the power button on top and visible beyond the edge of the clip (see Figure 6). The clip will help keep the laser steady during use.
   {12114_Procedure_Figure_6}
9. Place the frame on a level surface 1.5–3 meters away from a wall. Note: The further away the laser is from the wall, the more spread out the diffraction pattern will be. Measurements will be easier, but the diffraction pattern will be dimmer.
10. Place a piece of masking tape on the level surface to mark the position of the measurement frame.
11. Place a 2- to 3-cm thick book or other flat object directly behind the frame and place the laser on the book. The lens of the laser should point at the fishing line across the opening of the frame (see Figure 7).
    {12114_Procedure_Figure_7}
12. Tape a piece of white paper to the wall as a screen where the laser beam will shine when it is on.
13. Using a meter stick, measure the distance from the frame to the paper screen. Record the distance in millimeters in the data table on the Measuring with Laser Light Worksheet.
14. Place the tip of the laser pointer within 1 cm of the fishing line.
15. Holding the back of the binder clip, press the power button and hold it down to turn on the laser light. Caution: Never look directly into the beam of the laser—serious eye injury may result!
16. Aim the laser beam directly at the fishing line. When the laser is positioned correctly, a horizontal diffraction pattern of light and dark bands will be seen on the screen, with a brighter red spot in the middle (see Figure 8).
    {12114_Procedure_Figure_8}
17. Once a clear diffraction pattern is visible, one partner should quickly and carefully place the straight edge of a piece of masking tape on the screen at the center of a dark band near the middle bright spot, then place a second piece of tape at the center of the next dark band on the screen. The pieces of tape may be placed either to the right or the left of the middle spot, but not one on either side. The distance between the pieces of tape represent Δy (see Figure 8).
18. As soon as the pieces of tape have been placed, release the power button on the laser.
19. Write “fishing line” between the pieces of tape on the screen.
20. Repeat steps 14–18 with the copper wire, using the same distance from the measurement frame to the screen on the wall. Adjust the frame to the right or left if necessary to ensure the diffraction pattern is in a different place on the screen than before. Observe any difference in the diffraction pattern made by the copper wire from the pattern made by the fishing line. Write “copper wire” between the new pieces of tape.
21. Repeat step 20 with the human hair. Write “human hair” between the two new pieces of tape.
22. Remove the paper screen from the wall.
23. Complete the data table and answer the questions on the worksheet.

Student Worksheet PDF

12114_Student1.pdf