Which of the following phenomena happens because the refractive index n varies with wavelength?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A prism breaks sunlight into different colours
Option (1) A lens focuses light — Incorrect. A lens focuses light by refraction at its curved surfaces. This happens even for light of a single wavelength.
Option (2) A prism breaks sunlight into different colours — Correct. Glass has a different refractive index for each wavelength. So each colour in sunlight is deviated by a different angle, and the colours spread out. This splitting of light into its constituent colours is dispersion.
Option (3) Total internal reflection ensures that light travels down an optical fibre — Incorrect. Total internal reflection needs only a core of higher refractive index than the cladding. The angle of incidence must also be above the critical angle. It works with light of a single wavelength.
Option (4) Light rays entering a pond change direction at the pond’s surface — Incorrect. The bending is refraction, caused by the different speeds of light in air and water. It occurs for light of any single wavelength.
NCERT (2026-27):
Refraction – “… We note that n21 is a characteristic of the pair of media (and also depends on the wavelength of light), but is independent of the angle of incidence.”
Summary – “7. Dispersion is the splitting of light into its constituent colour.”