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Difficulty: EasyRefraction of Light, Total Internal Reflection, and Prisms

Which of the following conditions must be satisfied for light to undergo total internal reflection at the boundary between two transparent media?

  1. The light ray must travel from an optically denser medium into an optically less dense medium, and the angle of incidence must exceed the critical angle.Answer
  2. B
    The light ray must travel from an optically less dense medium into an optically denser medium, and the angle of incidence must exceed the critical angle.
  3. C
    The light ray must travel from an optically denser medium into an optically less dense medium, and the angle of incidence must be less than the critical angle.
  4. D
    The light ray must travel from an optically less dense medium into an optically denser medium at an angle of incidence equal to 0∘0^\circ.

Answer

The light ray must travel from an optically denser medium into an optically less dense medium, and the angle of incidence must exceed the critical angle.
Total internal reflection occurs only when light passes from a medium of higher optical density into a medium of lower optical density, and the angle of incidence exceeds the critical angle for that boundary.

Step-by-Step Solution

1
Identify the boundary requirement for wave propagation direction in total internal reflection.
Light must travel from an optically denser medium (higher refractive index n1n_1) toward an optically rarer medium (lower refractive index n2n_2).
This direction allows the ray to bend away from the normal, increasing the angle of refraction relative to the angle of incidence.
2
Determine the required angle of incidence at the boundary.
The angle of incidence ii must be strictly greater than the critical angle θc\theta_c (where sin⁡θc=n2/n1\sin \theta_c = n_2 / n_1).
When i>θci > \theta_c, no refraction can take place because sin⁡r>1\sin r > 1, forcing all light energy to reflect back into the initial denser medium.

Key Concept

Conditions for Total Internal Reflection
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Topics

Refraction of Light, Total Internal Reflection, and Prisms

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