physics

Name Date Class

Lab 28: Diffraction and Interference

Purpose

To study single slit diffraction and double slit interference patterns

Background

It has long been known that if you shine light through narrow slits that are spaced at small intervals, the light will form a diffraction pattern. A diffraction pattern is a series of light and dark areas caused by wave interference. The

wave interference can be either constructive (light areas) or destructive (dark areas). In this experiment, you will shine a laser through a device with two slits where the spacing can be adjusted and investigate the patterns that are

produced on the far side of the slits.

Skills Focus

Predicting, drawing conclusions, observing, interpreting data, making generalizations, applying concepts

Procedure

1. Start Virtual Physics and select Diffraction and Interference from the list of assignments. The lab will open in the Quantum laboratory.

2. A laser is used as the light source in this experiment because it has a single wavelength. Therefore, you will not see diffraction patterns from other wavelengths interfering in the image. What is the wavelength of the laser?

What is the spacing of the two slits on the two slit device? This is the gap

between the two different slits. How do the wavelength of the laser and the spacing of the slits compare?

3. Predicting How will the diffraction pattern change as the wavelength is

ade smaller and the slit spacing remains the same? Hint: Think about the spacing as an obstacle that the waves are running into.

Diffraction and Interference 89

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.

Name Date Class

4. Drawing Conclusions Observe the pattern displayed on the video screen

as you reduce the wavelength to 600 nm and then down to 300 nm by one- hundred nanometer increments. Click on the down arrow below the hundreds place to change these values. What can you state about the relationship between wavelength and diffraction pattern when the wavelength is greater than the obstacle?

5. Observing Now you will investigate other interference effects. Once you

spread out the slits farther, you can start to see interference when waves passing through the two different slits interfere with each other. Change the wavelength of the laser to 500 nm and the slit spacing to 3μm. Describe what you observe. What is causing this effect?

6. Change the intensity of the laser from 1 nW to 1W. Does the intensity of the

light affect the diffraction pattern?

7. Interpreting Data Change the slit spacing to 1 μm. Then observe the

pattern displayed on the video screen as you change the slit spacing from

1 μm to 7 μm by one-micrometer increments. What can you state about the relationship between slit spacing and diffraction pattern?

8. Return the slit spacing to 3 μm. Increase the wavelength of the laser to 700 nm.

What affect does an increase in the wavelength have on the interference pattern?

90 Diffraction and Interference

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.

Name Date Class

9. Making Generalizations Decrease the intensity on the laser to 1000

photons/second. Click on the Persist button (the button with a black arrow)

on the video camera to look at individual photons coming through the slits. Observe for one minute. What observation can you make about this pattern

as compared to the pattern from the continuous beam of photons?

10. Decrease the intensity to 100 photons/second. Observe for another minute

after clicking Persist. At these lower intensities (1,000 and 100 photons/ second), there is never a time when two photons go through both slits at the same time. How can a single photon diffract?

11. From this experiment, what conclusions can you make about the nature of

light?

12. Applying Concepts Click in the Stockroom. Click on the Clipboard and

select Preset Experiment 8: Two-Slit Diffraction—Electrons. Click the green

Return to Lab arrow. This setup is similar to the previous one, except that

the source is emitting electrons, rather than photons of light, so the detector

is a phosphor screen which can detect charged particles. How does this diffraction pattern compare to the diffraction pattern for light?

Diffraction and Interference 91

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.