1. Total Internal Reflection
When light travels between mediums, some light is reflected and some is refracted
Light bends away from the normal when it speeds up at the boundary of two media
Angle of refraction is larger than the angle of incidence

2. Critical Angle
The angle of refraction continues to increase as the angle of incidence increases.
Eventually, the angle of refraction will become 90º
At this point, the angle of incidence is called the CRITICAL ANGLE

3. It’s Trapped! Click HERE
If the angle of incidence is increases past the critical angle, the refracted ray will not exit the medium
It will reflect back into the medium
This is called TOTAL INTERNAL REFLECTION
Click HERE

4. Conditions for TIR
Total internal reflection occurs when 2 conditions are met:
Light is travelling more slowly in the first medium than in the second
The angle of incidence is larger than the critical angle

5. Calculating Critical Angle
Calculate the critical angle for a water air boundary.
n1=1.33
n2=1.00
Θ2 = 90º
θ1 = ?
n1sin(θ1) = n2sin(θ2)
sin(θ1) = n2sin(θ2) n1
sin(θ1) = (1.00)sin90
1.33
sin(θ1) = 0.75
θ1 = sin-1(0.75)
θ1 = 49°
The critical angle of the water air boundary is 49 °