Question

Define the term 'intensity of radiation' in photon picture of light.
Ultraviolet light of wavelength 2270 $$A^\circ$$ from 100W mercury source irradiated a photo cell made of a given metal. How would the photocell respond to a high intensity.$$(~10^5Wm^{-2})$$red light of wavelength $$6300 A^\circ$$ produced by a laser?

Answer 1

The intensity of light of certain frequency (or wavelength) is defined as the number if photons passing through unit area in unit time.
For a given wavelength,$$(\lambda)$$ of light
$$\dfrac{hc}{\lambda}=W+K$$
$$=W+eV_s$$(where $$V_s$$ is stopping potential)
$$\dfrac{6.63\times10^{-34}\times3\times10^8}{2270\times10^{-10}}=W+1.6\times10^{-19}\times(-1.3eV)$$
$$\therefore W=\left(\dfrac{6.63\times10^{-34}\times3\times10^8}{1.6\times10^{-19}\times6300\times10^{-19}}-1.3\right)$$
$$=\dfrac{6.63\times3}{1.6\times6.63}\times10=\dfrac{198..9}{1.6\times63}eV=1.973eV$$
The energy of red light is very less than its work functin, even intensity is very high.
Hence no emission of electron is possible.