posted by JJ .
for our lab, we had to fill a semi-circle plastic dish (with a scratch on one side) with water and find the index of refraction of water by putting a pin on one side of the dish, looking through the water, and put a pin on the other side of the dish to line up with the first pin and the scratch. we then found the angles of incidence and refraction. but the light goes from air to plastic to water. does the plastic pose a problem since it has its own index of refraction?
In going through the plastic side wall of the dish, a light ray will be displaced a small amount (comparable to the thickness of the wall) but will not change its direction. The plastic will not have a significant effect on the measurement.