# Transition of Electron

When an electron makes transition from higher energy level having energy

*E*_{2}(*n*_{2}) to a lower energy level having energy*E*_{1}(*n*_{1}), then a photon of frequency*Î½*is emitted

**Fig. 7**

# Energy of emitted radiation

**Frequency of emitted radiation**

# Wave number/wavelength

Wave number is the number of waves in unit length

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# Number of spectral lines

If an electron jumps from higher energy orbit to lower energy orbit, it emits raidations with various spectral lines.

If electron falls from orbit

*n*_{2}to*n*_{1}, then the number of spectral lines emitted is given byIf electron falls from

*n*th orbit to ground state (i.e.,*n*_{2}=*n*and*n*_{Â1}= 1) then number of spectral lines emitted# Recoiling of an atom

Due to the transition of electron, photon is emitted and the atom is recoiled.

Recoil momentum of atom = Momentum of photon

Also, recoil energy of atom (where

*m*= mass of recoil atom)