Materials with higher refractive indices have smaller critical angles than materials with lower refractive indices.

• A. Higher RI -> smaller critical angle
• B. Higher RI -> larger critical angle
• C. Critical angle independent of RI
• D. Critical angle equals RI value

Answer: (A)

This question tests how the critical angle depends on refractive indices through Snell’s law. When light travels from a denser medium (higher refractive index n1) to a rarer medium (lower refractive index n2), the critical angle θc satisfies sin θc = n2 / n1 (with total internal reflection occurring once the incidence angle exceeds θc).

As you increase the refractive index of the first medium while keeping the second medium the same, the ratio n2/n1 gets smaller. A smaller ratio means sin θc gets smaller, so θc itself decreases. That’s why higher refractive index materials have smaller critical angles.

For example, with n2 = 1.0 (air) and n1 = 1.5 (typical glass), θc ≈ arcsin(1/1.5) ≈ 41.8°. If n1 increases to 1.9, θc ≈ arcsin(1/1.9) ≈ 31°. The critical angle is determined by the ratio, not by the refractive index value itself, and it isn’t independent of RI nor equal to the RI value.