Fundamentals

The critical angle is the angle of incidence that results in an angle of refraction of 90°. When light is in the more dense material approaching the less dense material at the critical angle, light is refracted along the boundary line. For angles of incidence greater than the critical angle, total internal reflection occurs (there is no refraction). And for angles of incidence less than the critical angle, there is a little reflection and some refraction (but with angles of refraction less than 90°).
 

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

Version 1:

The three cases below show light traveling from a more dense to a less dense medium at varying angles of incidence. In one case, the angle of incidence (Θinc) is equal to the critical angle (Θcrit). In the other two cases, the angle of incidence (Θinc) is either greater than (>) or less than (<) the critical angle (Θcrit).

 

How to Think About This Situation

A Matching Exercise
You have to do some matching - matching the relationship beweeen the angle of incidence and the critical angle to the diagram showing either ...

  1. Total Internal Reflection (TIR).
  2. Both Reflection and Refraction (R&R) ... with the refracted ray being at a 90° angle of refraction and lying along the boundary line.
  3. Both Reflection and Refraction (R&R) ... with the refracted ray not being along the boundary line.



Matching Rules
You will need to be guided by the Rules identified in the Fundamentals section (above). They are ...

  1.  If Θincidence = Θcritical , then R & R occurs with the refracted ray lying along the boundary.
  2.  If Θincidence > Θcritical , then TIR occurs. There is no refracted ray.
  3.  If Θincidence < Θcritical , then R & R occurs. The refracted ray will be less than 90° from the normal.

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

Total Internal Reflection

The Critical Angle


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