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Q.
Assertion : The energy $(E)$ and momentum $(p)$ of a photon are related by $p=E / c$.
Reason : The photon behaves like a particle.
AIIMSAIIMS 2005Dual Nature of Radiation and Matter
Solution:
When an electron/proton behaves like a particle, then its wavelength is given by
$\lambda=\frac{h}{p}$ ...(1)
where $h$ is Planck constant, $p$ is momentum. This wavelength is know as de-Broglie wavelength of the electron. Also
$E=h v=\frac{h c}{\lambda}$ ...(2)
where $v$ is frequency, $c$ is speed of light.
From Eqs. (1) and (2), we get
$p=\frac{E}{c}$