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In the teaching materials for physics course, I have the following question:

enter image description here

I know the correct answer is B, and I can make arguments why based on the orthonormal projection against the prism.

However, I need a more intuitive answer for students. How could I explain this?

Thanks for the help! I'm using this forum to improve my methods :)

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3 Answers 3

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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.

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  • $\begingroup$ but the macroscopic laws of physics are not symmetrical with respect to time reversal! $\endgroup$
    – hyportnex
    Commented Sep 20, 2015 at 19:32
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You need to talk about light bending either towards or away from the normal, depending on whether it is going from a higher-to-lower refractive index, or vice versa. Drawing on the normals at each of the points where the ray of light hits a boundary will help them to understand this.

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  • $\begingroup$ I'll try this method and the above approach. Thank you! $\endgroup$
    – JianguoHisiang
    Commented Sep 20, 2015 at 22:09
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Use normals at the boundary.Light rays bend towards the normal as they enter a medium with high optical density(glass). They bend away from the normal as the enter a medium with low optical density(air).

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  • $\begingroup$ This argument does not eliminate D as a choice. Besides, it's the same as another posted answer. $\endgroup$
    – garyp
    Commented May 1, 2021 at 14:12

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