ν 0B Figure . Variation of stopping potential with frequency of the incident radiation for two metals Now a graph is drawn between frequency of incident radiation and the stopping potential for different metals (Figure . ). From this graph, it is found that stopping potential varies linearly with frequency.
Below a certain frequency called threshold frequency, no electrons are emitted; hence stopping potential is zero for that reason. But as the frequency is increased above threshold value, the stopping potential varies linearly with the frequency of incident light. . .
Laws of photoelectric effect The above detailed experimental investigations of photoelectric effect revealed the following results: i) For a given metallic surface, the emission of photoelectrons takes place only if the frequency of incident light is greater - - - - Unit dual nature of radiation and matter For the sake of simplicity, the following standard assumptions can be made when light is incident on the given material. a) Light is absorbed in the top atomic layer of the metal b) For a given element, each atom absorbs an equal amount of energy and this energy is proportional to its cross-sectional area A c) Each atom gives this energy to one of the electrons. (Given : The work function for cesium is . eV and the power absorbed per unit area is Wm which produces a measurable photocurrent in cesium.) Solution i) According to wave theory, the energy in a light wave is spread out uniformly and continuously over the wavefront.
The energy absorbed by each electron in time t is given by E = IAt With this energy absorbed, the most energetic electron is released with K max by overcoming the surface energy barrier or work function ϕ and this is expressed as K IAt max = − ϕ ( ) Thus, wave theory predicts that for a unit time, at low light intensities when IA < ϕ , no electrons are emitted. At higher intensities, when IA ≥ ϕ , electrons are emitted.