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Refraction of Light

Total internal reflection and the critical angle

  • 3 things to know
  • Component 3

Overview

Two conditions required for total internal reflection and critical angle.

What you need to know

Answer each one before you reveal it.

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Key term

State the two conditions required for total internal reflection.

Key term

Define the critical angle.

Key term

Give the critical angle equation for a boundary with air.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    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 must exceed the critical angle.

  2. 2

    Define the critical angle.

    Model answer

    The critical angle is the angle of incidence in the denser medium at which the angle of refraction is ninety degrees.

  3. 3

    Derive the equation for the critical angle at a boundary between two media.

    Model answer

    Putting the angle of refraction equal to ninety degrees in n₁ sin θ₁ = n₂ sin θ₂ gives n₁ sin C = n₂, since sin90° is one.

  4. 4

    Give the critical angle equation for a boundary with air.

    Model answer

    sin C = 1/n, where n is the refractive index of the denser medium.

  5. 5

    Calculate the critical angle for glass of refractive index 1.50 in air.

    Model answer

    sin C = 1 ÷ 1.50 = 0.667, so C is 41.8°.

  6. 6

    Check that critical angle by substituting it back into Snell's law.

    Model answer

    1.50 × sin41.8° = 1.50 × 0.667 = 1.00, which is 1.00 × sin90°, as the definition requires.

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