A hydrogen atom moving at a speed v absorbs a photon of wavelength 122 nm and stops. Find the value of v. Mass of a hydrogen atom = 1.67 x 10–27 kg.

energy of a photon is = hc/λ

visible light lies in 390-750 nm 

maximum energy for 390 nm

h=6.626 x  10^-34 

c= 3 x 10⁸

λ=390 x 10^-9

E=5.09 x 10^-19 Joules

Use the formula     lambda = h/mv

lambda = 127nm

h = 6.626*10^-34 Js

mass (m)= 1.67*10^-27 kg

Now evaluate do it as a practice ...

You get v =3.1 m/s

hope it clears