I. Rays of light travel from a less dense medium to a denser medium. II. The Angle of incidence is greater…

Analyze the question

You are asked which statements give the conditions needed for total internal reflection (TIR).

Concept

Total internal reflection happens when light is completely reflected back into the denser medium at the boundary. Two conditions must both be met:

1. The light must travel from a denser medium to a less dense medium (e.g. glass to air, or water to air).
2. The angle of incidence must be greater than the critical angle (c) for that pair of media.

The critical angle is the angle of incidence in the denser medium for which the angle of refraction is 90°. It is given by sin c = 1/n, where n is the refractive index of the denser medium.

Checking each statement

• I. Light travels from a less dense to a denser medium. False. Here the ray bends towards the normal, so there is no critical angle and TIR cannot occur.
• II. Angle of incidence is greater than the critical angle. True. This is a required condition.
• III. Light travels from a denser to a less dense medium. True. This is the other required condition.

So the correct statements are II and III.

Answer: C. II & III only

Option D (II only) is incomplete. A large angle of incidence alone is not enough, because the light must also be going from the denser medium into the less dense one. Both conditions are needed.

Exam tip: Remember "Dense to Less, Greater than Critical", meaning from the denser medium to the less dense medium, with the angle of incidence greater than c. If a statement says light goes into a denser medium, eliminate it straight away for TIR questions.