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Dual Nature of Radiation and Matter — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsDual Nature of Radiation and Matter3 Marks
Question
  1. How does one explain the emission of electrons from a photosensitive surface with the help of Einstein's photoelectric equation?
  2. The work function of the following metals is given: Na 2.75 ev, K = 2.3 eV, Mo = 4.17 eV and Ni = 5.15 eV. Which of these metals will not cause photoelectric emission for radiation of wavelength 3300 Å from a laser source placed 1 m away from these metals? What happens if the laser source is brought nearer and placed 50 cm away?
✓

Answer

  1. Einstein’s Photoelectric equation is

$hv=\varphi_0+K_{\text{max}}$

When a photon of energy $'hv'$ is incident on the metal, some part of this energy is utilised as work function to eject the electron and remaining energy appears as the kinetic energy of the emitted electron.

  1. $E=\frac{hc}{\lambda}=\frac{6.63\times10^{-34}\times3\times10^8}{3.3\times10^{-7}\times1.6\times10^{-19}}eV$

$=3.77\text{eV}$

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

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