Question

A monochromatic light passes

Answer 1

Answer:

The way I see this problem is that if I'm sure how the light should enter the prism then, it must be the same way the other way around.

You can argue that the laws of physics are the same under time reversal. Or use the symmetry of the prism and the entering beam.

You can also use a more intuitive way of thinking. If you are standing on the poolside and you shot a laser beam on the floor you expect to have to look in the same direction where the laser is pointing in order to see the point at the bottom of the pool. Here you can show how the light take the same path in the two ways.