Identify the correct statement among the following:

The correct answer is Option (4) → The rest mass of a photon is zero.

In the photon picture of radiation, light is a stream of massless, uncharged particles, each of energy \( h\nu \), all moving at speed c in vacuum.

Option (1) Intensity of any light ray does not depend on the number of photons present in it. — Incorrect. For light of a given frequency, the intensity is the number of photons crossing unit area per second times \( h\nu \), so it depends directly on the number of photons.

Option (2) In free space, the speed of photons emitted by a source of radiation depends on the frequency of radiation. — Incorrect. All photons travel at \( c = 3\times10^{8}\,\mathrm{m\,s^{-1}} \) in free space whatever their frequency; frequency decides their energy, not their speed.

Option (3) Photons are deflected by electric or magnetic fields. — Incorrect. Photons carry no charge, so electric and magnetic fields do not deflect them.

Option (4) The rest mass of a photon is zero. — Correct. A photon moves at c, which is possible only for a particle of zero rest mass; its momentum \( h\nu/c \) comes from its energy alone.