Question

For a given pair of transparent media, the critical angle for which colour is maximum?

• A. Red
• B. Blue
• C. Violet
• D. Green

Answer 1

The Correct Option is A

The critical angle (\(C\)) for total internal reflection is given by the relation: 

\[ \sin C = \frac{1}{\mu} \]

where \( \mu \) is the refractive index of the denser medium relative to the rarer medium.

Important: Refractive index (\(\mu\)) varies with wavelength (colour) due to dispersion. For visible light, the refractive index is highest for violet (shortest wavelength) and lowest for red (longest wavelength).

Step-by-Step Logic:

• Red light has the longest wavelength, thus the smallest refractive index (\(\mu\)).
• As refractive index decreases, the critical angle increases, because:

\[ \sin C = \frac{1}{\mu} \]

Therefore, among visible colours, red will have the smallest refractive index, and thus the maximum critical angle.