In Hydrogen atom , if an electron jump from n2 level into n1 level . Prove that the wave number (reciprocal of wave length) of emitted radiation is :

Question

In Hydrogen atom , if an electron jump from n2 level into n1 level . Prove that the wave number (reciprocal of wave length) of emitted radiation is : 
1/λ=me4 /8ε02h3c (1/n22   -1/n12)