What happens when two light beams interfere?

The correct answer is Option (3) → Energy is redistributed.

Option (1) Energy is lost in the process. — Incorrect. No energy is lost. The energy missing from the dark fringes appears in the bright fringes. So the total energy is conserved.

Option (2) The light beams bend. — Incorrect. Interference is the superposition of the two beams where they overlap. It does not change their direction. Bending into the region of shadow is diffraction.

Option (3) Energy is redistributed. — Correct. For two equal coherent sources the intensity is \( 4I_0 \) at the bright fringes and zero at the dark fringes. The light energy is shifted from the dark regions to the bright regions, with no gain or loss in total.

Option (4) Energy is uniformly distributed. — Incorrect. A uniform intensity \( 2I_0 \) at all points is what incoherent sources give. Interference produces alternate maxima and minima of intensity.

NCERT (2026-27):
Seeing the Single Slit Diffraction Pattern – “In interference and diffraction, light energy is redistributed. If it reduces in one region, producing a dark fringe, it increases in another region, producing a bright fringe. There is no gain or loss of energy, which is consistent with the principle of conservation of energy.”