1. The problem statement, all variables and given/known data My question is very basic, yet one which has become increasingly frustrating. I am doing an investigation into determing the refractive index of Perspex for an assignment. To do this I am using a light ray box to shine a ray onto a rectangular perspex prism, and then using a pencil to trace out the path of the light ray. Then using a protractor to find the angles of incidence and refraction...before putting it into snell's law to give me the refractive index. My question is What would the controlled variables be in this experiment??? 2. Relevant equations BTW, I'm using the equation: sin i/sin r = n to find refractive index (being n in this case) 3. The attempt at a solution Back to my problem. The only controlled variable I can come up with is the temperature of the air; as cold and hot air density varies. I cannot think of anymore and I need to clearly state them as part of this assignment. Any ideas? Thanks for any help. This is a wonderful forum which I'm sure I will be using in the future as I'm doing highschool physics through distance education.