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Physics Honors/AP 1 and 2

LAB: Angles of Refraction and Focal Lengths

Course Materials are always under revision! Weblecture content may change anytime prior to two weeks before scheduled chat session for content.

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Physics Lab

Physics Lab: Multiple Slit Diffraction

Goal: Determine the wavelength of light using diffraction gratings



  1. Set up the ruby laser so that it shines along the normal to the flat screen, approximately 20-40cm from the screen.
  2. Hold the diffraction grating 1-2 cm in front of the laser. Note the locations of the maxima and minima.
  3. Move the ruby laser further from the screen without moving the diffraction grating. Do the maxima locations change? If so, how?
  4. Hold the ruby laser further steady and move the diffraction grating toward and away from the screen. Do the maxima locations change? If so, how?
  5. Fix the ruby laser and diffraction grating so that at least two maxima show on either side of the central maximum.
  6. Measure the distance y1 to the first maxima on each side. If the distances are not the same, verify that your laser is pointing along the normal to the screen, and that the screen is flat and at right angles to the laser beam.
  7. Measure the distance y2 to the second maxima on each side.
  8. Measure the distance L between the diffraction grating and the screen at the location of the central maximum.

Data Handling

  1. Use the formula for slit diffraction mλ = d sin θ = d Δy/L and calculate the wavelength for both the first and second maxima.
  2. The standard ruby laser wavelength is 694.3nm. Determine the amount of error in your calculation.


Write a formal lab report, including your materials and procedure. Present your data in tabular form and show your calculations for wavelength and error handling.

Post your report to the Lab thread at the Moodle.