Physics II AP

The Groovy CD Experiment

 

Background

The grooves in a compact disc are very close together. One side of a disc can hold more music than two sides of a vinyl record. Just how close are the grooves? When you look at the side with the grooves in it you see a rainbow spectrum. The rainbow spectrum is from the reflection of white light that has been diffracted.

To measure the distance between the grooves we will inspect the diffraction pattern from a laser with a known wavelength. Fortunately, we know how to analyze the pattern. The equation for the groove separation distance d may appear intimidating at first:

d = n L sin A where
  • n is order of the bright spot in the pattern
  • L is the wavelength of the light reflecting from the CD
  • A is the angle of orientation of the bright spot.

 

 

This angle is determined by measuring the distance x from the CD to the "screen" (in this case, the mounted meterstick) and the distance y of the first or second order bright spot from the central (zero order) bright spot.

A bit of clever trigonometry turns x and y into an angle

Specifically,

A = tan-1 (y/x)

So by knowing the wavelength of our laser light-for the helium-neon laser we are using-and by measuring the distances x and y, we are able to determine the distance between the grooves on a compact disc.

Procedure

When a beam of light reflects from the "groovy" surface of a CD, a diffraction pattern is produced. To those who know how to analyze it, this pattern reveals the distance between the grooves.

First mount the CD as shown by your instructor. Next place the laser at least two meters away from the CD and point it toward the CD. DO NOT TURN ON THE LASER UNTIL EVERY THING IS SET UP AND READY. Finally set up a meter stick to measure the diffraction pattern. A diagram of the arrangement is shown below.

 

 

 

Turn on the laser and measure the distance from the appropriate distances.

So by using your data from Part I of the lab for the wavelength of our helium-neon laser (632.8 nM) and by measuring the distances x and y, we are able to determine the distance between the grooves on a compact disc.

Analysis

  1. Make the x and y measurements for at least two orders and carry out the calculations for the angles.
  2. Determine the distance between the grooves on a compact disc. Show all work.
  3. The actual distance is 1.6 microns. What is your percent error for each order?

Questions? Comments?
Michael Gilmore