Question

Physics question!

Answer 1

Could you please explain about those red areas? Thank you.

Answer 2

The top red area is showing the angle of incidence, with respect to the normal, that resulted in total internal reflection.

The second red area is showing the angle of incidence from within the substance, and then the angle from which it leaves the substance, both with respect to the normal.

It appears that you were supposed to find these angles. Using the index of refraction of the substance (and that of air), we can use Snell's Law to find the internal angle of a ray incident on the surface at 45 degrees (28 degrees).

Given that the ray did reflect, and the geometry of the medium (a rectangle), we can get the top angles just using what we already know to this point. We can form a triangle out of the ray and the normals of the two sides like this:

Since the angles of a triangle sum to 180 degrees, we can solve for the other angle of this triangle (62 degrees).

Within a single medium, the angle of reflection equals the angle of incidence, so that gives us the next angle. Using another triangle, we can solve for the angle of incidence on the boundary of the other side. Then use Snell's law again to solve for the angle the light ray leaves the substance. Due to symmetry, these are all the same as what we've already calculated.