I am writing a paper on "Photoelastic Stress Test by Polarization" and let me tell you that it is not at university level. It is my Extended Essay for IB -if you know what IB is. I am looking for a simple explanation on why these colorful stress patterns form. So far I found out that when object is under stress it creates some local differences at the index of refraction of object so at different parts of the object, light travels at different speeds and bends more or less. But I dont understand why we get colors.