Complete the worksheet from Beyond Labz on Diffraction and Interference, and then Write one half to two pages of report discussing these question.
To study single slit diffraction and double slit interference patterns.
Have you ever played with making waves in a bathtub or watched the waves on the beach? When a
water wave runs into an obstacle, what happens? Diffraction patterns are really interesting effects that
occur when waves interfere with each other. You can observe the same type of interference patterns with
light waves or sound waves.
Diffraction, interference
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 patterns caused by
wave interference. The wave interference can be either constructive (light patterns) or destructive (dark
patterns).
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 will be made at a distance from the slits.
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 and you will not
see diffraction patterns from other wavelengths interfering in the image. Look at the controller on the
side of the laser to note the wavelength of the light. Record the wavelength in Question 1.
3. There is a device in the middle of the table that has two adjustable narrow openings to let the light
through. Record the gap between the two different slits in Question 2.
4. Observe the current diffraction pattern and predict how the diffraction pattern will change as the
wavelength changes in Question 3.
5. Change the wavelength to 600 nm. Observe the pattern displayed on the video camera screen as you
change the wavelength from 700 nm to 300 nm by one-hundred nanometer increments. Record your
observations in Question 4.
6. Now you will investigate other interference effects. Once we spread out the slits farther, we 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 3 μm. Record your observations in
Question 5.
Name ___________________________________________ Date ______________________ Class ________________________
7. Change the slit spacing to 1 μm. Observe the pattern displayed on the video screen as you change the
slit spacing from 1 μm to 7 μm by one-micrometer increments and record your observations of the
pattern you observe in Question 6.
8. Return the slit spacing to 3 μm. Increase the wavelength of the laser to 700 nm. Record your
observations of how an increase in the wavelength changes the interference pattern in Question 7.
9. Change the intensity of the laser from 1 nW to 1 W. Observe the diffraction pattern and record your
observations in Question 8.
10. Decrease the intensity on the laser to 1000 photons/second. Click on the Persist button on the video
camera to look at individual photons coming through the slits. Observe for one minute. Decrease the
intensity to 100 photons/second. Observe for another minute. 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.
Record your observations in Question 9.
1. What is the wavelength of the laser?
2. What is the spacing of the two slits on the two-slit device? How do the wavelength of the laser and the
spacing of the slits compare?
3. How will the diffraction pattern change as the wavelength is made smaller and the slit spacing remains
the same?
4. What can you state about the relationship between wavelength and diffraction pattern when the
wavelength is greater than the obstacle?
5. Describe what you observe when the slit spacing is changed. What is causing this effect?
Name ___________________________________________ Date ______________________ Class ________________________
6. What can you state about the relationship between slit spacing and diffraction pattern?
7. What affect does an increase in the wavelength have on the interference pattern?
8. Does the intensity of the light affect the diffraction pattern?
9. What observations can you make about the individual photon pattern as compared to the pattern from
the continuous beam of photons?