[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. http://img518.imageshack.us/img518/5184/apparatusgt4.png [Broken] 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.