When light travels from a high-index, optically dense slow medium into a low-index, optically less dense faster medium, the ray bends away from the normal.  If the angle of refraction reaches or exceeds 90°, the beam is unable to escape the high-index medium.  This phenomenon is called  total internal reflection  and it can be demonstrated using the Light Refraction simulation.
total internal reflection : the reflection of a wave that is travelling from a  high-index  medium into a  low-index  medium at an angle equal to or greater than the critical angle

 

Using the   Refraction  simulation, set the refractive index of the top layer to equal 1.33 for water and the refractive index of the bottom layer to equal 1.00 for air.  Position the laser pointer in the top layer near the left side and project it downward (as shown below).


  1. Complete Table 3 by rotating the laser pointer to the angles specified and calculating the angle of refraction using Snell's Law.  (The equation has been manipulated to solve for the angle of refraction.)  Note: Refraction will not occur for all the angles in the table.


 

θ 1

θ 2

35°

 

 

40°

 

 

45°

 

 

50°

 

 

55°

 

 

     

  1. What is the maximum possible angle of incidence that still causes refraction?  What happens to the ray of light if the angle of incidence exceeds this value.

The maximum possible angle of incidence that will still cause refraction is known as the  critical angle .  The critical angle can be calculated by assuming the angle of refraction is 90°.  At this angle, the ray is refracted parallel to the interface between the mediums.  Any increase in the incident angle will cause the refracted ray to no longer refract but to bounce, or reflect, back into the higher index medium.
critical angle : for any two mediums, the size of the incident angle that causes the angle of refraction to be 90°