Hi there, When performing the Young's double slit expt, we always pass the light source through a single slit first before the actual double slits. The typical textbook answer for the use of the first slit is to "produce 2 coherent light sources at the 2 slits", or "to ensure a constant phase difference between waves from the 2 double slits". If the light we are using is white light, i can understand the use of the first slit. However, I do not understand its significance if we are using single color lasers. For example, if we are using laser light which is monochromatic, arent all the waveforms already in phase? In other words, straight from the laser source to the double slits, the waveforms will still be in phase. Can fellow forummers shed some light pls?