Total Internal Reflection


The previous pages illustrated light traveling from a rarer medium (air) to a denser medium (water).  You saw that light bends toward the normal in this situation, and that the angle in the denser medium will be smaller than the angle in the rarer medium.  Now we'll look at what happens when light travels from a denser medium like glass into a rarer medium like air.
 
Figure A: Light bends away from the normal when traveling into a rarer medium.
  1. When light travels from a denser medium to a rarer medium, it bends away from the normal, as illustrated to the left.  This behavior follows from Snell's Law.  As you saw before, the larger angle must be in the medium having the lower index of refraction.  On the previous pages, that was medium 1, but here it is medium 2.
Figure B: When the angle of incidence equals the critical angle, the angle of refraction is 90 degrees.
  1. Since the angle in the second medium is greater than the angle in the first medium, it can become as large as 90 degrees.  When this occurs, the first angle is called the critical angle and is represented as θc. 
Figure C: When the angle of incidence is greater than the critical angle, no light exits the glass.
  1. If the angle θ1 in the first medium is larger than the critical angle, no light makes it into the second medium.  This condition is called total internal reflection (TIR).  Note that some reflection occurs for all angles of incidence θ1, but once θ1 becomes larger than θc, all of the light is reflected from the surface.  Also be aware that total internal reflection and total reflection are not equivalent expressions.

 
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Can Snell's Law be used to predict when TIR will occur?
Continue to the next page to find out!
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