Which of the following is NOT true?

The correct answer is Option (3) → In Young’s double-slit experiment, the bright fringes become wider as we move farther from the central maximum.

Option (1) Sound waves are longitudinal waves. — True. Sound in air travels as compressions and rarefactions. The air particles oscillate along the direction of propagation, so sound is a longitudinal wave.

Option (2) Heat waves (infrared waves) are transverse waves. — True. Infrared waves, also called heat waves, are electromagnetic waves. Their electric and magnetic fields are perpendicular to the direction of propagation, so they are transverse.

Option (3) In Young’s double-slit experiment, the bright fringes become wider as we move farther from the central maximum. — Not true. The bright fringes lie at \( x_n=n\lambda D/d \), so the gap between neighbouring fringes, \( \lambda D/d \), is the same for every \( n \). The fringes are equally spaced, and they do not widen away from the centre.

Option (4) Light waves can travel through vacuum. — True. Light is an electromagnetic wave. A changing electric field produces a magnetic field, and a changing magnetic field produces an electric field. So the wave needs no material medium and travels even in vacuum.

NCERT (2026-27):
Interference of Light Waves and Young’s Experiment – “We will have constructive interference resulting in a bright region when \( \displaystyle\frac{xd}{D}=n\lambda \). That is,

    \( \displaystyle x=x_n=\frac{n\lambda D}{d};\ n=0,\pm1,\pm2,\ldots \qquad (10.13) \)

… Such bands are called fringes. Equations (10.13) … show that dark and bright fringes are equally spaced.”