Diffraction: Measuring CD Groove Size.

With a monochromatic light source and a CD it is possible to create a diffraction pattern. If a diffraction pattern is created from incident light on a CD surface, then the groove thickness in the CD can be measured. In this lab a red laser and a CD was used to create a diffraction pattern.  By projecting the pattern on to a screen and measuring the appropriate geometry, the value of groove thickness was obtained.

Image of CD surface depicting grooves and pits.

Diffraction in action! Neat!

Diffraction pattern created by CD.

The dots on the whiteboard correspond to intensity maxima. The order of the maxima are denoted and the geometry used is show in the triangle.

The distance measured from the CD and the screen is denoted by L in the above image. The distance from maximum to maximum was measured and denoted by X in the above image. From the geometry theta was calculated to be 1.46 degrees. The equation used to find the length of CD grooves was:

where m is 1 and d is the groove thickness. Its worth noting that the uncertainty in the geometric measurements can be disregarded. The uncertainty of 0.05 cm to the maxima measurements and the measurement from the screen to the CD is relatively small in comparison to the actual measurements. The actual thickness of data tracks on a given CD is 740 nm.


When the calculations are done the calculated value for thickness is 16811 nm. This error is due to how the experiment was treated. Theta was measured incorrectly. If theta was less than one a small angle approximation could be used resulting in a small multiple of wavelength. For example (670/.90)=1.11*670=740.  This leads to the conclusion that the geometry was incorrectly measured.