(a)[2]
State what the term photon means.
(b)[4]
A samarium-157 nucleus ($^{157}_{62}\text{Sm}$) is initially stationary and emits a gamma-ray ($\gamma$-ray) photon with energy $0.57\ \text{MeV}$. Determine, for a single $\gamma$-ray photon: (i) the wavelength, (ii) the momentum.
(c(i))[2]
Use your result from (b)(ii) to determine the speed of the samarium-157 nucleus after the photon has been emitted.
(c(ii))[1]
Using your answer in (c)(i), explain quantitatively why the speed of the samarium-157 nucleus can be regarded as negligible when compared with the speed of the photon.