Q: With what velocity must an electron travel so that its momentum is equal to that of a photon with a wavelength of 5000 A°

(h = 6.6 × 10^{-34} Js ,m_{e} = 9.1 × 10^{-31} Kg)

Sol: $\large m v = \frac{h}{\lambda}$

$\large v = \frac{h}{m \lambda} $

$\large v = \frac{6.62 \times 10^{-34}}{9.1 \times 10^{-31} \times 5000 \times 10^{-10}} $

v = 1450 m/s