Evans SPH 3U1

Physics Grade 11

Unit 3: Light and Optics

Note 5: Total Internal Reflection and the Critical Angle


Reference: Chapter 11.5

Demo: Laser with glass show total internal reflection


Example 1: What happens when light tries to travel from plastic (n=1.60) to air if the angle of incidence is 60 degrees?

(try to solve and show how no refraction exists -  we have total internal reflection)


Examine Fig. 11.20 in text pg. 368


Critical Angle:

The incident angle is called the critical angle when the refracted angle is exactly 90 degrees

Total Internal Reflection:

Total Internal Refelection occurs when the angle of incidence increases past the critical angle.

Total Internal Reflection can occur when light travels from a more optically dense medium to a less optically dense medium (not the other way - show with diagrams)

It is when the angle of incidence is increased to a point where there is no longer any refraction and the ray is now just reflected.


Use Snell's Law to find the critical angle, where subscript r will be for the less optically dense medium.

                           For the critical angle problems and


Example 2: Calculate the critical angle for a plexiglass -air boundary. The index of refraction for plexiglass is 1.51.


Perform Optics Lab for Snell's Law and Total Internal Reflection (see handout and text pg. 397-398)