[SOLVED] Optical absorption in Semiconductors 1. The problem statement, all variables and given/known data I'm doing a lab report on the optical absorption of semiconductors. More specifically it was to determine the bandgap values of the semiconductor samples. This is the experimental setup. One of the questions which was asked was why a chopped light source was used instead of a constant incident light source. I can't quite figure out why. 3. The attempt at a solution I did some searching on the internet and found that chopping the light source allows one to convert a dc light signal into an ac (pulsed) signal, and the relevant ac signal can be extracted by using a locked-in amplifier. But I still don't understand why a dc light source can't be used instead.