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.

The plastic dish that you used in your lab does have its own index of refraction, but it should not pose a significant problem for your measurements. When light passes through the plastic side wall of the dish, it will be displaced by a small amount due to the refractive index of the plastic. However, this displacement will not change the direction of the light ray.

In your experiment, the important part is the path of light from air to plastic to water. The light undergoes refraction at both the air-plastic interface and the plastic-water interface. By measuring the angles of incidence and refraction, you can determine the index of refraction of water.

To account for the plastic, you need to consider the angle of incidence at the air-plastic interface and calculate the angle of refraction using the index of refraction of the plastic. Then, take this refracted ray to the plastic-water interface and measure the angle of incidence and refraction once again.

By comparing the angles of incidence and refraction at both interfaces, you can calculate the index of refraction of water. The plastic dish itself should not introduce a significant error as long as you take into account the refractive index of the plastic when analyzing your measurements.

It's always a good practice to consult your lab manual or instructor for specific guidance on how to account for any possible sources of error or uncertainties in your experiment.