Interference and Diffraction

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Double Slit Theory:

The setup is that the two sources of coherent rays are separated by a distance d. The screen where the waves are observed is so far away that the two rays are effectively parallel.

Use the space below to show that bright spots are found at angles defined by


Also show that bright spots will appear on a screen located a distance R away at locations defined by





  • Double and Single slits
  • Diffraction grating
  • Laser pointers (red, green, violet, blue, red-green)

Part 1: Double slit interference

Shine the green laser (λ = 532 nm) through the narrow double slit and project the pattern on a screen.

Be sure to keep both the slits and the screen perpendicular to the optical axis (light path). For best results, separate the screen and slits by the largest possible distance.

Double Slit Setup




Optical bench

Sketch and describe the observed pattern in the space below. In your description, be sure to use the words “constructive”, “destructive”, “wavelength” and “pathlength difference”.

Measure the spacing between the central maximum and another maximum. For best results, make use of the symmetry, and measure to a large value of m.

Double slit green.jpg

Δy = _________ mmax/min= _______________ So your measured ym is y_ =________

Which order? ↑

Measure the distance between the screen and the slit.

L = ______________________

Are you justified in using the small angle approximation? Y/N Why?

Use the measured values and the nominal laser wavelength (on the body of the laser) to determine the slit spacing.

a= __________________

Part 2: Repeat Part 1 with the same slit, but a different colored laser (λ = 405 ± 10 nm).

Double slit blue.jpg

What happened to the spacing of the pattern? Why?

Record your data. Clearly label your parameters.

Show your work to calculate the slit separation.

Do your answers from Part 1 and Part 2 agree to within a reasonable degree? Estimate some error to verify. Explain any assumptions.

Part 3: Attempt with the third laser pointer (λ = 655 ± 25 nm). Show your work if possible.

Double slit redgreen.jpg

Again, do your answers for the slit separation agree to within a reasonable degree with each other? With the nominal spacing on the slides?

For which data are you most confident? Why?

Part 4: Hair width

Remove the double slit completely and place a strand of hair in the path of the beam.

Why is a double slit pattern observed?

Measure the width of the hair. Use the most appropriate laser for best precision. Show your work.

Hair data cropped.jpg

Part 5: Single slit.


In this setup, the width is still much smaller than the distance to the screen, and the rays, all ∞ of them, are still parallel. Use the space below to clearly show




Materials: Single slit, laser pointer, screen, mounts, ruler, meter stick

Choose a laser for precise results (remember your double slit experiments). Shine the laser light through the single slit and sketch the pattern in the space below.

Single slit A.jpg

Single slit B.jpg

Single slit C.jpg

Measure the width of the central fringe, 2y1:___________________

What is the screen-slit distance, R? _____________ The laser wavelength λ? _______

Show your calculations to find the slit width a.


How does it compare with the value indicated? Estimate the largest sources of error and decide whether your measured value is within a reasonable range of the nominal (listed) value of a.

Part 6: The diffraction grating.

Remove the single slit. Make sure your laser pointer is aimed at the wall, away from other groups. Add a transmission grating. Be careful about stray beams – the small angle approximation is generally a very BAD approximation for diffraction gratings!

Diffraction Grating – Red Green setup

Diffraction grating redgreen data.jpg

How many orders do you see? That is, what’s the maximum value of m? _____

If you switch to another color of laser pointer, should you see more or fewer orders? Explain your prediction, below.

Now try it out. For extra clarity, mount both laser pointers simultaneously so they have a slight vertical offset, but the m=0 beams are lined up.

Diffraction Grating Green – right.jpg

Diffraction Grating Green – left.jpg

Was your prediction correct? (Y/N) If not, explain any misconceptions.

Let’s get quantitative. Turn off the laser pointer. Set up a CD in a three-fingered clamp such that it is nearly vertical. Aim the laser pointer at it, almost along the normal to the CD, so that the back reflections pass just over the laser pointer and hit the wall. You will want to maximize the CD-wall separation while not causing stray reflections to bounce all over the room. Sketch the geometry below, labeling all measured distances.

CD setup.jpg

Your diffraction pattern may not be perfectly symmetric.

CD A.jpg

CD B.jpg

Find the maximum value of m for which you can clearly observe both positive and negative diffraction spots. Measure the distance, then divide by 2 to account for small asymmetries.

m=______ Distance between order pairs: ________ Average ym=_________

What’s the CD-wall distance? ____________

Show your calculations to determine the groove spacing on a CD.

Distance between adjacent grooves: __________

Let’s do a check for reasonability. If the laser can resolve the distance between adjacent grooves, presumably it can also resolve the difference between pits of roughly the same spacing. Let’s calculate how much data we should be able to fit on the disk. Measure the active area of the disk. Sketch and record data below.

If we assume roughly square pits, how many such squares could you fit in the active area? Each pit is one “bit” of digital information. There are 8 bits in 1 byte. How many bytes can you fit on the disk? Show your calculation.

Experimental capacity: ________

Compare to the standard capacity of a CD. This may be listed on the disk, or you can search for a value and cite your source.

Accepted capacity: _____________ Source:

Discuss any discrepancies.

If time allows, repeat the experiment with a DVD-R. Show your data and calculations on extra paper as needed.

Explain the pros and cons of a Blu-Ray disk, using the information you have learned in this lab and about diffraction more generally.

Of the three experiments (double slit, single slit, and diffraction grating), which one came closest to the predicted values? Why do you think that is?

Last Updated on May 4, 2020 by Essay Pro