When a metal plate is exposed to a monochromatic beam of light of wavelength 400 nm, a negative potential of 1.1 V is needed to stop the photocurrent. Find the threshold wavelength for the metal.
Given, wavelength 
Potential (i.e.; stopping potential) =1.1V
From Einstein’s photoelectric equation,
⇒ 
Where 
is the threshold wavelength,
is the wavelength of light,
h is Planck’s constant,
c is the speed of light,
e is the charge on electron,
is the stopping potential
⇒
⇒ 
⇒ 
⇒ 
∴ 
(threshold frequency) =
m.
1