The photoelectric effect was not observed for
Potassium
Rubidium
Lithium
Caesium
As learnt in
Photoelectric Effect -
The electrons are ejected from the metal surface as soon as the beam of light of a particular frequency strikes the surface.
-
Lithium has a very high ionization potential on account of its very small size thus making photoelectric effect difficult.
Option 1)
Potassium
The option is incorrect.
Option 2)
Rubidium
The option is incorrect.
Option 3)
Lithium
The option is correct.
Option 4)
Caesium
The option is incorrect.