Photoelectric effect - Practice Questions & MCQ

Whenever a metal surface is exposed to light radiation of suitable energy or frequency, it was observed that some electrons get ejected from the metal surface. This phenomenon is called as Photoelectric effect and the ejected electrons are called as photoelectrons.

There were certain observation in the photoelectric effect experiment. 

(1) There was a requirement of a minimum energy for each metal for the photoelectric effect to occur. This minimum energy is known as work function (W₀) and it can be closely associated with the ionisation energy of the metal.

• Corresponding to the work function, there is a minimum frequency of light required for photoelectric effect. This minimum frequency is called as Threshold frequency.
• Corresponding to the work function, there is a maximum wavelength of incident light above which Photolelectric effect cannot occur. This maximum wavelength is called as the Threshold wavelength.

Mathematically, the work function, threshold frequency and threshold wavelength can be associated as 

$\mathrm{W}_0=\mathrm{h} \nu_0=\frac{\mathrm{hc}}{\lambda_0}$

Note: hc is approximately equal to 2 x 10^-25 J-m or 12400 eV-nm. (eV is the energy in electron volts)

(2) The number of electrons ejected is proportional to the intensity (brightness) of light striking the metal but does not depend upon the frequency of light. 

(3) There was almost no time lag between the striking of light and ejection of photo electrons

(4) The kinetic energy of the ejected electrons (photoelectrons) depend upon the frequency of the light used.

Einstein's photoelectric equation

From conservation of energy

$\begin{gathered}E_p=W_0+K E \\ \therefore h \nu=h \nu_0+\frac{1}{2} m v^2\end{gathered}$

where

m is the mass of the electron

v is the velocity associated with the ejected electron.

h is planck’s constant.

v is frequency of photon,

v₀ is threshold frequency of metal.

(5) The Kinetic energy of ejected photoelectron is also sometimes associated to Stopping Potential. It is defined as the minimum opposing potential applied due to which kinetic energy of electron becomes zero.

$\frac{1}{2} m v^2=e V_s$

where,

V_s = Stopping potential 

e = Charge on electron