Question

The critical angle of glass-air interface is (given ${\mu }_{glass}=1.5$):

• A. ${24}^o$
• B. ${48}^o$
• C. ${42}^o$
• D. ${45}^o$

Answer 1

Answer: (C)

When light travels from one medium to another it changes speed and is refracted. If the light rays are travelling for a less dense material to a dense medium they are refracted towards the normal and if they are travelling from a dense to less dense medium they are refracted away from the normal.
For total internal reflection to occur the light must travel from a dense medium to a less dense medium (e.g. glass to air or water to air).
As the angle of incidence increases so does the angle of refraction. When the angle of incidence reaches a value known as the critical angle the refracted rays travel along the surface of the medium or in other words are refracted to an angle of 90 degrees. The critical angle for the angle of incidence in glass is 42 degrees.