Question 14: The work functions of Caesium (Cs), Potassium (K) and Sodium (Na) are 2.14 eV , 2.30 eV and 2.75 eV respectively. If incident electromagnetic radiation has an incident energy of 2.20 eV , which of these photosensitive surfaces may emit photoelectrons?
(1) K only
(2) Na only
(3) Cs only
(4) Both Na and K
Answer: Option (3)
Explanation:
According to the photoelectric effect, photoelectrons are emitted only when the energy of the incident radiation is greater than or equal to the work function of the metal.
E \ge \phiThe given energy of incident radiation is
E = 2.20 \, \text{eV}The work function of Caesium is
\phi_{\text{Cs}} = 2.14 \, \text{eV}Since
2.20 \, \text{eV} > 2.14 \, \text{eV}photoelectrons will be emitted from Caesium.
The work function of Potassium is
\phi_{\text{K}} = 2.30 \, \text{eV}Since
2.20\,\text{eV}<2.30\,\text{eV}photoelectrons will not be emitted from Potassium.
The work function of Sodium is
\phi_{\text{Na}} = 2.75 \, \text{eV}Since
2.20\,\text{eV}<2.75\,\text{eV}photoelectrons will not be emitted from Sodium.
Hence, only Caesium can emit photoelectrons.