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Home Concept Builders Refraction and Lenses Total Internal Reflection Help Help 3

Total Internal Reflection - Questions 3 Help

Total internal reflection (TIR) involves the reflection of all of the light that approaches the boundary with another material. There are two requirements for TIR:
  1. The angle of incidence must be sufficiently large (i.e., greater than the so-called critical angle).
  2. The incident light must be in the more dense material heading towards the least dense material.

There are three similar versions of this question. Here is one of those versions:
 

Version 1:

For the three situations below, the relative index of refraction (n) of Medium 1 and 2 is shown. In which situation(s) could the light ray undergo total internal reflection (assuming that the angle of incidence were sufficiently large)?

Know the Rule
You must determine which of the three light rays will undergo total internal reflection. There are two requirements; you can read about them in the Fundamentals section (above). The first requirement is met since the question states that we can assume that the angle of incidence is sufficiently large. And so you must focus on the second requirement, selecting as answers any of the situations in which light is in the more dense medium and heading towards the least dense medium.

Making Sense of "More Dense"
The problem would be easier if the relative density of the two media were given. But they aren't. Instead the relative index of refraction value of the two media is given. We know that the index of refraction value provides a measure of how optically desnse a material is. So the materials that are most dense are those with the largest n value. And so instead of looking for the more dense to less dense situations, you need to be looking for the large n to small n situations. Go one by one through the three diagrams and select all that show the incident light being in the medium with the greatest n value. Those situations will be included among your answers.

Try the links below to our Tutorial for more information about total internal reflection:

Total Internal Reflection

The Critical Angle

Concept Builder Teacher Notes Directions