Why no refraction at the curved side in Snell's law lab

[uchicago2012]

Homework Statement

In a lab we did, we verified Snell's Law of Refraction by measuring the index of refraction for water. We filled a plastic half-circle with water and drew a line normal to the flat side. We measured an angle 30 degrees from the normal line (called the incident ray). We then looked at a pin on this incident ray through the water and aligned pins with it on the curved side. I attached our procedure in case none of that makes sense. But I was wondering why we didn't do refraction on the curved side?

Homework Equations

Snell's Law: n₁sin(θ₁) = n₂sin(θ₂)

3. The attempt at the solution
I thought it might be because any line we draw to the curved side would be a normal line and thus there would be no refraction?

 

[gneill]

Any ray that originates in at the center of the flat side that passes through the water and exits the curved side is following a radius of the arc. What does that tell you about the angle of incidence of the ray with the curved side (from the water side)?