[College Physics II: Waves/Optics] Index of Refraction

[AgentRedfield]

Homework Statement

A solid glass cube with edge length of 10.0 mm and index of refraction n=1.75 has a small, dark spot dead center of the cube. Find the minimum radius of black paper circles that could be pasted at the center of each cube face to prevent the center spot from being seen, no matter what the direction of viewing.
Picture (link)

Homework Equations

Snell's Law (maybe?)
n₁/n₂=sin(θ₂)/sin(θ₁)

The Attempt at a Solution

I'm really confused where to even start. A hint in the right direction is all I'm hoping for. Thank you for your time.

 

[Simon Bridge]

Hint: total internal reflection.

 

[AgentRedfield]

Hint: total internal reflection.

I was able to figure it out. Thank you!

 

[Simon Bridge]

Well done and no worries :)