Total internal reflection occurs in an optical prism when light rays

Analyze the question: We're asked for the condition under which total internal reflection (TIR) occurs in a prism.

Concept: Total internal reflection needs two conditions together:

1. Light must travel from an optically denser medium to a less dense medium (e.g., glass to air).
2. The angle of incidence in the denser medium must be greater than the critical angle (c).

The critical angle is given by:

sin c = 1/n, where n is the refractive index of the denser medium relative to the less dense one.

For glass with n = 1.5, sin c = 1/1.5 = 0.667, so c ≈ 42°. This is why 45° prisms (like those in periscopes and binoculars) can reflect light totally.

Evaluating each option:

• A. Travel from the prism into an optically denser medium. Wrong. Going into a denser medium bends light toward the normal, so TIR cannot happen.
• B. Are incident in the prism at an angle greater than the critical angle. Correct. This is the defining condition for TIR, and it assumes the light is in the denser prism, meeting a less dense boundary.
• C. Are incident normally in the prism. Wrong. Normal incidence means an angle of 0°, so the light passes straight through without bending or reflecting totally.
• D. Travel from the prism to the air. Partly true, since it is the right direction. But it is incomplete: light going from glass to air at an angle smaller than the critical angle simply refracts out. The angle condition must also be met, so D alone is not sufficient.

Answer: B

Exam tip: Remember "Denser to Less dense, Angle bigger than c" (DLA). If an option mentions the critical angle being exceeded, it is usually the best answer for TIR questions. Direction alone is never enough.