Measuring CD Track Spacing

In this lab the goal was to find the spacing of tracks of a regular CD. To do so we used a laser and we pointed it at the CD and the reflection was observed on a whiteboard. The setup was as follow:

The result from this setup looked like this:

Even though the dots are clear the curving in the pattern makes the geometry harder. The curving happens due to the fact that the CD is round, so a CD grating is different compare to the regular gratings used in class. For this reason we changed our setup in following manner:















This way the diffraction pattern would not curve as much and everything can easily be measured with a meter stick.

The formula for the diffraction is:

where θ is the angle between zeroth maxima to the first maxima, λ is the wavelength of the red laser, m is the number of the maxima and d is the grating which is what we were looking for.

y=27±0.5cm
λ=650±20nm

x= m=17±1cm

m=1

since this value equals 32.2 degrees we cannot use small angle approximation. After plugging in the proper values the value obtained for spacing is 1220

which equals 1220±0.5 μm


however the value given for a track spacing is about 1600 μm. If we take into account the uncertainty maximum value for the spacing with our value is 1332 μm which is still off by about 16% from 1600 μm
this shows that there is error somewhere in the experiment that we hav'nt accommodated for, or the geometry and setup of the experiment is faulty.