What is total internal reflection?

Total internal reflection is the complete reflection of a wave back into the denser medium at a boundary, with no light refracting out, when the angle of incidence inside that medium exceeds the critical angle.

Define the critical angle.

The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the less dense medium is exactly 90°.

State the equation for the critical angle at a boundary between a material and air, defining each symbol.

sin C = 1/n, where C is the critical angle measured from the normal inside the material and n is the refractive index of that material.

CCEA A-Level Physics 1210

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Refraction

Total internal reflection and the critical angle

3 things to know
Unit AS 2

Overview

Total internal reflection and critical angle.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the equation for the critical angle at a boundary between two materials, defining each symbol.

Key term

State the two conditions required for total internal reflection.

Numbers

Calculate the critical angle for a glass–air boundary where the glass has a refractive index of 1.50.

Questions from this subtopic

Each one with the model answer underneath.

1
What is total internal reflection?
Model answer
Total internal reflection is the complete reflection of a wave back into the denser medium at a boundary, with no light refracting out, when the angle of incidence inside that medium exceeds the critical angle.
2
State the two conditions required for total internal reflection.
Model answer
The light must be travelling from a medium of higher refractive index towards one of lower refractive index, and the angle of incidence at the boundary must be greater than the critical angle.
3
Define the critical angle.
Model answer
The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the less dense medium is exactly 90°.
4
State the equation for the critical angle at a boundary between a material and air, defining each symbol.
Model answer
sin C = 1/n, where C is the critical angle measured from the normal inside the material and n is the refractive index of that material.
5
State the equation for the critical angle at a boundary between two materials, defining each symbol.
Model answer
sin C = n₂/n₁, where n₁ is the refractive index of the denser medium the light is travelling in, n₂ is that of the less dense medium beyond the boundary, and C is the critical angle.
6
Calculate the critical angle for a glass–air boundary where the glass has a refractive index of 1.50.
Model answer
sin C = 1/1.50 = 0.667, so C = 41.8°, and 1/sin 41.8° returns 1.50 as given.

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