(such as photon-electron collision), the total energy and total momentum are conserved. However, the number of photons may not be conserved in a collision. The photon may be absorbed or a new photon may be created. Example .
Monochromatic light of frequency . Hz is produced by a laser. The power emitted is . – W.
(a) What is the energy of a photon in the light beam? (b) How many photons per second, on an average, are emitted by the source? Solution (a) Each photon has an energy E = h + = ( . – J s) ( .
Hz) = . – J (b) If N is the number of photons emitted by the source per second, the power P transmitted in the beam equals N times the energy per photon E , so that P = N E . Then N = . W .
J P E = . photons per second. Example . The work function of caesium is .
eV. Find (a) the threshold frequency for caesium, and (b) the wavelength of the incident light if the photocurrent is brought to zero by a stopping potential of . V. Solution (a) For the cut-off or threshold frequency, the energy h + of the incident radiation must be equal to work function - , so that + = .14eV .
J s Thus, for frequencies less than this threshold frequency, no photoelectrons are ejected. (b) Photocurrent reduces to zero, when maximum kinetic energy of the emitted photoelectrons equals the potential energy e V by the retarding potential V . Einstein’s Photoelectric equation is