What is the momentum of a photon of energy 3 MeV?

The correct answer is Option (1) → \( 1.6\times10^{-21}\,\mathrm{kg\,m\,s^{-1}} \)

In the photon picture of light, a photon of energy \( E=h\nu \) carries momentum \( p=h\nu/c \), so \( p=E/c \). Taking \( 1\,\mathrm{eV}=1.6\times10^{-19}\,\mathrm{J} \) and \( c=3\times10^{8}\,\mathrm{m\,s^{-1}} \), the energy in joules is:

    \( \begin{aligned} E &= 3\,\mathrm{MeV} \\ &= 3\times10^{6}\times1.6\times10^{-19}\,\mathrm{J} \\ &= 4.8\times10^{-13}\,\mathrm{J} \end{aligned} \)

The momentum of the photon is:

    \( \begin{aligned} p &= \frac{E}{c} \\ &= \frac{4.8\times10^{-13}\,\mathrm{J}}{3\times10^{8}\,\mathrm{m\,s^{-1}}} \\ &= 1.6\times10^{-21}\,\mathrm{kg\,m\,s^{-1}} \end{aligned} \)

NCERT (2026-27):
Particle Nature of Light: the Photon – “… (ii) Each photon has energy E (=hν) and momentum p (= hν/c), and speed c, the speed of light.”