Groovy CD Experiment

Groovy CD Experiment

Equipment Needed

- Laser

- Compact Disc

- Meter Stick

- Screen

- Diffraction grating (known number of slits/mm)

Purpose

To determine the space between grooves in a compact disc.

Introduction

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 white light that has been diffracted and interferes.

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 diffraction expression introduced in class was:

m l = Y

d X

where: m is the order number of the observed fringe

l is the wavelength of the laser light

d is the distance between grooves on the disc.

Y is the distance between the central zero order fringe and the mth order fringe

X is the distance from the CD to the screen

Procedure

1. Set up a CD with the side that is read facing towards the desk

surface. The CD now will act as a reflection grating rather than a transmission grating.

2. Aim the laser upwards from underneath the disc so that the light reflects back downwards.

Angle the disk slightly so that the spots can be seen clearly on the desk surface.

3. Measure Yleft and and Yight. Average them to get Y

Yleft = ______m Yright = ______m Y = ______m

Measure the distance (X) from the CD to the screen. X = ______m

4. Use Y, X , l and the diffraction expression to determine d (m)

Work: d (m) = ______m

5. The industry standard for the space between tracks on a CD is 1.6 microns (1.6 x 10-6 m).

What is the percentage error in your calculation of this distance?

% error = Actual distance – Calculated distance x 100 %

Actual distance

Work: % error = ______

Extension

Try to determine the track spacing for a DVD or Blue Ray Disk. Compare your answers with a CD. Do you think using a green or a blue laser would improve your results? Explain!