Experiment 12: CD Diffraction

Purpose:
The purpose of this experiment is to use the concept of light interference and diffraction when a light beam goes through a grating to measure the distance between the grooves for a CD and a DVD. When the grooves spacing for a CD and a DVD are experimentally found, the results will be compared with the actual values are computed by the factory. The standard grooves spacing for a  CD is 1600nm and DVD is 740nm.

Procedure:
1. Obtained a meter stick, a laser beam, a CD, a DVD, and a screen with a small center hole.
2. Set a laser beam pointing through the hole of a screen to the CD or DVD.

3. Adjusted the disks so that the zero order maximum reflected light strikes through the hole of the screen and the first order maxima appeared on the screen.

Data Analysis for the CD:

	λ (nm)	Distance (l)  between the screen and the disk (cm)	Distance (x) between the  hole and the bright fringe (cm)
Trial 1	633	32.4 ± 0.5	14.8 ± 0.5
Trial 2	633	34.8 ± 0.5	16.0 ± 0.5
Average	633	33.6 ± 0.5	15.4 ± 0.5

Results:

Experimental grooves spacing (nm)	Standard spacing  (nm)	% error	% uncertainty error
1519 ± 112	1600nm	5 %	7 %

Data Analysis for the  DVD:

	λ (nm)	Distance (l)  between the screen and the disk (cm)	Distance (x) between the  hole and the bright fringe (cm)
Trial 1	633	5.5 ± 0.3	9.1 ± 0.3

Results:

Experimental grooves spacing (nm)	Standard spacing  (nm)	% error	% uncertainty error
739 ± 89	740nm	0.14 %	12 %

Sample calculations:

how to calculate the grooves spacing

 uncertainty calculation

Summary:
According to the results, the % error between the experimental grooves spacing of the CD and DVD comparing to the standard values are 5% and 0.14% respectively. These % error are within experimental uncertainties. The error could be from the scratch on the surface of the disks and some uncertainties of the measurement.
The grooves spacing depends on the wavelength of the laser light they use to shine the disk. Therefore, the shortest wavelength they can use now is the the gamma ray which should give the spacing in order of

10^-12 m. One way to improve the capacity of a DVD or CD is to add more layers. However, these layers have to have different pitch so the incoming light will not be destructive interference.