CHAPTER 11 DUAL NATURE OF RADIATION AND MATTER
- Work function: The minimum energy needed by an electron to come out from a metal surface is called the work function of the metal. Energy greater than the work function required for electron emission from the metal can be supplied by suitable heating or applying strong electric field or irradiating it by light of suitable frequency.
- Photoelectric effect: Photoelectric effect is the phenomenon of emission of electrons by metals when illuminated by light of suitable frequency. Certain metals respond to ultraviolet light while others are sensitive even to the visible light.
- Photoelectric effect involves the conversion of light energy into electrical energy. It follows the law of conservation of energy. The photoelectric emission is an instantaneous process and possesses certain special features.
- Photoelectric effect depends on:
- the intensity of incident light
- the potential difference applied between the two electrodes
- the nature of the emitter material
- The stopping potential (V0) depends on:
- the frequency of incident light
- the nature of the emitter material
- Below a certain frequency (threshold frequency), characteristic of the metal, no photoelectric emission takes place, no matter how large the intensity may be.
- The classical wave theory could not explain the main features of the photoelectric effect. Einstein explained these features on the basis of photon picture of light. According to this, light is composed of discrete packets of energy called quanta or photons. Each photon carries an energy E= h ν and momentum p = h/λ which depend on the frequency of the incident light and not on its intensity.
- Einstein’s photoelectric equation is in accordance with the energy conservation law as applied to the photon absorption by an electron in the metal.
0.5 m v2max = V0 e
- Radiation has dual nature: wave and particle. The nature of the experiment determines whether a wave or particle description is best suited for understanding the experimental result.
- The waves associated with the moving particles are called matter waves or de Broglie waves.
- The de Broglie wavelength (λ) associated with a moving particle is related to its momentum as λ = h/p. the de Broglie wavelength is independent of the charge and the nature of the material particle.
- Electron diffraction experiments by Davisson and Germer and by G.P Thomson have verified and confirmed the wave nature of electrons. The de-Broglie hypothesis of matter waves supports the Bohr’s concept of stationary orbits.