Electromagnetic waves of wavelenglth `1242 Å` are indicdent on a metal of work funcation `2eV`. The target metal is connected to a `5` volt cell, as shown. The electrons pass through hole A into a gas of hydrogen atoms in their groynd stte. Find the number of spectral lines emitted when hydrogen atoms come back to their froynd states after gaving veen excited by the electrons, Assume all excitations in H-atoms from ground state only (`hc = 12420 eVÅ`)

Answer

6

Explanation

`kE_(max) = (12420ÅeV)/(1242Å) - 2eV = 8 eV`

The electrons are emitted with kinetic energy varying from zero to `8 eV`
When acclerated with `5` volt potential difference their energies increases by `5eV`.
Hence hydrogen will get photons of energies in the range from `5` eV to `13 eV`.
So maximum possible transactions are upto `n = 4`, Hence number of spectra lines is `.^(4)C_(2) = 6`