Experiment 12: CD Diffraction

Introduction

                The purpose of this experiment is to test a DVD to know whether the distance between the grooves causes distortion to the music. The groove spacing will be computed by employing the reflection-grating concept. The standard groove spacing for a DVD is 740 nm*. The experiment was conducted using a compact disc, a meter stick, a laser pointer, stands, and wood blocks. A compact disc was set up in front of the laser pointer so the laser beam would fall on the disc surface perpendicularly. The disc was adjusted so that the zero order maximum would strike the hold of the laser pointer. The first order maxima were found by putting two wood blocks, aligned with each other on each side of the laser pointer. The perpendicular distance between the screen (wood blocks) and the disc was recorded. The distance between two bright fringes on each side of the laser was also measured. The angle from the zero order maximum to the first order maximum was computed. The distance between the grooves was also calculated, and this value was compared with the standard value.

Figure 1: A compact disk was set up to remain perpendicular to the laser beam

 Figure 2: Laser was adjusted so that the zero order maximum stroke the hole of the laser

 

Data and Analysis

Table 1: Measured and computed data required for the grooves distance calculation

λ (nm)	Distance between the CD and the screen (cm)	Distance between 2 adjacent bright fringes (cm)	Angle between perpendicular line to the CD & the first bright fringe (˚)
632.8	11.70 ± 1.00	19.65 ± 1.00	59.23 ± 0.04

Table 2: Comparison between the experimental and the standard distance between the grooves

Experimental width (nm)	Standard width (nm)	% error (%)	% uncertainty (%)
736.5 ± 19.2	740	0.48	2.6

Uncertainty calculation

Conclusion

                According to table 2, the percent error between the experimental distance between the grooves and the standard values were 0.48% with the experimental distance of 736.5nm. In addition, the standard value was within the uncertainty of experimental error since the percent error was a lot smaller than the percent uncertainty. The error that could possibly contribute to this experimental result was that scratches on the disk and the uncertainties in measurement of the distances.

In order to improve the capacity of the DVD, the DVD can be added more layers. However, these layers have to have different pits so that the incoming waves will not interfere destructively when they reflect from each layer.

*Groove spacing on a DVD