MCAT Snell Law

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5+ Year Member
May 24, 2017
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I got the question right but I am not sure the specifics of why I would choose this answer. It was more of a gut response.

At what incident angle (θi) does a ray have to first hit the collection face to end up traveling directly up the fiber (up the page in Figure 1)?

A. 0° Correct Answer

B. 45°

C. 60°

D. 90°


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Nothing wrong with a gut answer. In fact, those are often the very best answer, because it shows you have an intuitive understanding of the scenario.

In the optical device shown, for a photon to travel straight up the fiber, it must pass through the first interface (referred to here as the collection face) and the reflect off of the second interface (referred to as the cleaved face.) To reflect straight up requires a total pathway change of 90˚ from start to finish. If the photon strikes the reflecting surface at 45˚, then the reflected angle is 45˚, which combine to equal 90˚, the exact value you need total. This means that the light cannot change angle when passing through the first interface (its refracted angle must equal its incident angle), which can only occur if it strikes the first interface at 90˚. Because incident angles are measured with respect to the normal, that makes the incident angle 0˚.

Hopefully this helps support what you already know deep inside.