Experiment 12: CD diffraction

Using the properties of diffraction, it is possible to measure the space between adjacent grooves on a CD. If you shine a laser on a CD and observe the spacing between the central maxima and the resulting accessory maxima it is possible to calculate the groove spacing by using the following geometric relationship.

Photo credits to sciwebhop

the distance between points A and B is notated as d. Θ is measured in degrees from the normal to the diffraction grating, and λ is the wavelength of the light incident on the diffraction grating. The following equation relates these variables.

d*sin(Θ)= m * λ

 Where m is the number of the maxima counting away from the central maxima.

When a laser is shone upon a CD the reflected pattern may appear something like this. 

It is clear from this image that some form of diffraction is taking place. By measuring the spacing between these maxima, and measuring the distance from the wall to our CD we can determine the angular separation of these maxima, additionally we know the wavelength of the laser to be 630-680 nanometers.

This table holds all of the relevant data.

 table

It is clear from the calculations that there is some sort of systematic error in our data collection, and it most likely stems from the fact that we were not measuring the appropriate phenomena. 

The predicted groove spacing based on our data is 2.6 * 10^-5 meters which is 16 times larger than the specifications listed for a CD(1.6 micrometers).

Unfortunately, we were unable to collect data after determining the flaw in our experiment and are unable to verify the CD groove spacing with the current data.

Additionally, here is an interesting video on another method for determining the groove spacing.

http://www.youtube.com/watch?v=IKKFPtcaZpQ