CBSE Class 12 Physics 2017 Delhi Set 1 Paper

Question : 14

Total: 26

(i) How does one explain the emission of electrons from a photosensitive surface with the help of Einstein's photoelectric equation?
(ii) The work function of the following metals is given : Na=2.75‌eV,K=2.3‌eV, Mo=4.17‌eV and Ni=5.15‌eV. Which of these metals will not cause photoelectric emission for radiation of wavelength 3300Å from a laser source placed 1m away from these metals? What happens if the laser source is brought nearer and placed 50‌cm away?

Solution:

(i) Explanation of emission of electrons from the photosensitive surface
(ii) Identification of metal/s which does/do not cause photoelectric effect
photoelectric emission Effect produced
(i) Einstein's Photoelectric equation is hv=ϕ0+Kmax
When a photon of energy ' hv ' is incident on the metal, some part of this energy is utilized as work function to eject the electron and remaining energy appears as the kinetic energy of the emitted electron.
(ii) E‌=‌

=‌

6.63×10−34×3×108

3.3×10−7×1.6×10−19

‌eV
‌=3.77‌eV
The work function of Mo and Ni is more than the energy of the incident photons; so photoelectric emission will not take place from tzhese metals. Kinetic energy of photo electrons will not change, only photoelectric current will change.

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