Two coherent sources of light can be obtained from

The correct answer is Option (4) → two slits illuminated by light from the same single source

Two sources are coherent when the phase difference between their waves at any point does not change with time.

Option (1) two different lamps — Incorrect. Light from an ordinary lamp undergoes sudden phase changes in times of the order of \( 10^{-10}\,\mathrm{s} \). So two independent lamps have no fixed phase relationship. The intensities on the screen just add up.

Option (2) two different lamps of the same colour — Incorrect. The same colour gives light of about the same frequency. But each lamp still changes its phase at random, independently of the other. So the phase difference does not stay fixed.

Option (3) two different lamps of the same colour and the same intensity — Incorrect. Equal intensity only makes the amplitudes equal. The phases of two separate lamps still change at random, so no steady interference pattern forms.

Option (4) two slits illuminated by light from the same single source — Correct. The light from both slits comes from the same original source. So any sudden phase change in the source shows up equally at both slits. The two slits keep a fixed phase difference and act as coherent sources, as in Young’s experiment.

NCERT (2026-27):
Interference of Light Waves and Young’s Experiment – “… S₁ and S₂ then behave like two coherent sources because light waves coming out from S₁ and S₂ are derived from the same original source and any abrupt phase change in S will manifest in exactly similar phase changes in the light coming out from S₁ and S₂.”