inquiry lab determining the refractive index of a variety of materials (Snell's Law Lab):

Question

Basically this lab:
We us a polar coordinate paper and a semicircular shaped containers which we switch between, which are composed of different material. We shine a narrow light through at angles up to 70, going up increments of 10 and measure the refraction angles.

I completed everything but I have two questions.

1.) Why is it impotent that the incident ray contacts the semicircular dish at the center? ( Can you explain referencing Snell's Law?)

2.) Why is it important that a "semicircular" dish is used , instead of a rectangular dish?

Answer 1

a) the refracted ray will hit the outer curved surface at a normal angle, preventing reflected and refracated internal ray.