Module 5 : MODERN PHYSICS
Lecture 24 : PHOTONS
  Example 8
  Consider a light source such as a laser with a power output of 1mW spread over a narrow beam of cross section 0.1 cm $ ^2$ falling on a surface of a metal. Estimate the time lag of photoelectron emission as per wave theory.
Solution
 
Taking atomic diameter to be of the order of $ 10^{-8}$ cm, the area exposed to the beam is $ 10^{-16}$ cm $ ^2$. The fraction of light energy absorbed is $ 10^{-16}/0.1 = 10^{-15}$. Thus the energy absorbed from the beam every second is $ 10^{-15}\times 10^{-3} = 10^{-18}$ J, which is approximately 6 eV (1 eV = $ 1.6\times 10^{-19}$ J). The amount of energy required to ionize an atom by dislodging an electron is typically 10 eV. Thus it takes about 1.6 seconds to absorb the required energy which is rough estimate of the time lag.

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