Atoms question bank (Physics)

An electron of a stationary Hydrogen atom transitions from the fifth energy level to the ground level. The velocity that the atom acquires as a result of photon emission is ( $m=$ mass of the atom,$R=$ Rydberg's constant,$h=$ Planck's constant).

Electrons of mass $m$ with de-Broglie wavelength $\lambda$ fall on a target. The cut-off wavelength $\lambda_0$ is equal to [$h=$ Planck's constant,$c=$ velocity of light].