Energy of photon is given by E=λhc ...(i)
Where h is the Planck's constant, c the velocity of light and λ its wavelength.
wavelength of the photon is given by λ=ph
where p is being momentum of the photon.
From Equations (i) and (ii), we get p=cE
Or E=p×c
Given, E=1MeV=1×106×1.6×10−19J c=3×108ms−1
Hence, after putting numerical values, we obtain p=3×1081×106×1.6×10−19 =5.3×10−22kgms−1