Question Answers

Photons

1. What is the threshold frequency for photoelectric emission? Does it depend on the intensity of light?

The minimum frequency of incident radiation required to liberate an electron from the metal surface is called threshold frequency. Photoelectric effect is not possible below this frequency whatsoever may be the intensity of light or time of exposure. Its value depends on the nature of the material and does not depend on the intensity of light.

2. Human skin is relatively insensitive to visible light, but ultra violet radiation can cause severe burns. Does this have anything to do with photon energies? Explain.

Energy of a photon of frequency f is given by, $$ E=hf $$ $$ i.e., E \propto f $$

Since, the frequency of ultra violet radiation is more than that of visible light, an ultra violet photon is more energetic then visible light photon. Hence, ultra violet radiation on the skin causes severe burns.

3. If we go on increasing the wavelength of light incident on a metal surface, what changes take place in the number of electrons and energy of the electrons?

The kinetic energy of electrons emitted from a metal surface when a light of wavelength λ is incident on it is given by, $$ KE=hf-\phi_o $$ $$ i.e., KE = h\frac{c}{\lambda}-\phi_o $$

This is Einstein's photoelectric equation. Since (h,c, \phi ) is constant, the wavelength of incident light increases as the energy of the electrons emitted decreases.

The number of electrons emitted depends on the intensity of incident light on the metal surface and is independent of frequency or wavelength of incident light. Hence, the number of electrons emitted does not change while increasing wavelength of incident light.