Electron are emitted from an electron gun at almost zero velocity and are accelerated by an electric field `E` through a distance of `1.0m` The electron are now scattered by an atomic hydrogen sample in ground state what should be the minimum value of `E` so that red light of wavelength `656.5 nm` may be emitted by the hydrogen?

Explanation

Energy corresponding to red light
`=(1242)/(lambda(nm))eV`
`=(1242)/(656.3)=1.9 eV`
This energy difference corresponds to n=3 to n=2. ltbygt The hydrogen should be excited from n=1 to n=3, so that it can emit n=3 to n=2.
`Delta E "for" n=1 "to" n=3 is 12.1 eV`
Energy of electron =eEd=eE
eE=12.1 eV
`E=12.1 V//m`