Question

Light from a hydrogen discharge tube is incident on a cds crystal (eg= 2.42 ev). Which spectral line from the balmer series (656 nm, 486 nm, 434 nm, 410 nm) are absorbed and which are transmitted ?

Answer 1

From photoelectric effect,
$K.E=E_0-\textbf{work function}$
so, energy of absorbed spectral line should be greater than work function e.g., 2.42eV

use formula, E = 1240/wavelength (nm) eV
= 1240/656 = 1.89 eV < 2.42eV
so, 656nm is not possible to absorb.

E = 1240/486 = 2.551eV > 2.42 eV
hence, 486 nm is possible to absorb.

similarly, 434nm, 410nm are also possible to absorb.

now, transmitted wave : 486nm , 434nm, 410nm