Chapter 28, Problem 1CQ

To determine

The reason for which the electrons left in the metals in an ordinary room without undergoing photoelectric effect.

Answer to Problem 1CQ

The threshold wavelength for photoelectric effect to take place is generally shorter than the wavelength of visible light. Moreover the coatings and impurity substances on the surface of the metal prevents the action of knocking out of electrons from the surface of the metal.

Explanation of Solution

In photoelectric effect, electrons are ejected from certain metallic surfaces as the electromagnetic radiation falls in it. The energy of the incident photon is utilized for two purposes in this process. One is to eject the electrons from the metal surface and the other is to provide kinetic energy to the ejected electron. The incident photons must have a threshold energy to knock out electrons from the metal surface. If the incident photons have wavelength higher than the threshold value, or frequency less than the threshold value, the knocking out of electrons will not occur.

In an ordinary room atmosphere, the ambient light is in the visible range, which have wavelength higher than the threshold wavelength. Thus, the energy of the incident photons of visible spectrum cannot knock out electrons due to the lack of sufficient energy. Moreover, the oxide layers and other coatings on the surface of the metal often hinder the interaction of the incident light and the electrons in the metal surface. These are the reasons for which the electrons left in the metals in an ordinary room without undergoing photoelectric effect