What does Young’s double-slit experiment demonstrate?

The correct answer is Option (1) → Interference of light

Option (1) Interference of light — Correct. In Young’s experiment the two pinholes S₁ and S₂ are lit by the same source, so they act as coherent sources. Their waves superpose on the screen and give alternate, equally spaced bright and dark fringes. This is the interference of light.

Option (2) Diffraction of light — Incorrect. Diffraction is the spreading of light from a single narrow slit or edge into the region of shadow. It gives a broad central maximum with weaker bands on either side. The equally spaced fringes of Young’s experiment come from the superposition of the two waves.

Option (3) Photoelectric effect — Incorrect. The photoelectric effect is the emission of electrons from a metal surface when light of high enough frequency falls on it. It shows the particle nature of light and involves no fringes.

Option (4) Black-body radiation — Incorrect. Black-body radiation is the radiation given out by a body because of its temperature. Young’s set-up studies how light from two coherent pinholes combines on a screen.

NCERT (2026-27):
Introduction – “… However, when Thomas Young performed his famous interference experiment in 1801, it was firmly established that light is indeed a wave phenomenon.”