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Dual Nature of Radiation and Matter 3. Photoelectric current depends on (i) the intensity of incident light, (ii) the potential difference applied between the two electrodes, and (iii) the nature of the emitter material. 4. The stopping potential (V ) depends on (i) the frequency of incident o light, and (ii) the nature of the emitter material. For a given frequency of incident light, it is independent of its intensity. The stopping potential is directly related to the maximum kinetic energy of electrons emitted: 2 e V0= (1/2) m v max = Kmax. 5. Below a certain frequency (threshold frequency) , characteristic of 0 the metal, no photoelectric emission takes place, no matter how large the intensity may be. 6. The classical wave theory could not explain the main features of photoelectric effect. Its picture of continuous absorption of energy from radiation could not explain the independence of K on max intensity, the existence of oand the instantaneous nature of the process. 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 ) and momentum p (= h/ ), which depend on the frequency ( ) of incident light and not on its intensity. Photoelectric emission from the metal surface occurs due to absorption of a photon by an electron. 7. Einstein’s photoelectric equation is in accordance with the energy conservation law as applied to the photon absorption by an electron in 2 the metal. The maximum kinetic energy (1/2)m v max is equal to the photon energy (h ) minus the work function 0 (= h 0 of the target metal: 1 m v 2 = V e = h – = h ( – ) 2 max 0 0 0 This photoelectric equation explains all the features of the photoelectric effect. Millikan’s first precise measurements confirmed the Einstein’s photoelectric equation and obtained an accurate value of Planck’s constant h. This led to the acceptance of particle or photon description (nature) of electromagnetic radiation, introduced by Einstein. 8. Radiation has dual nature: wave and particle. The nature of experiment determines whether a wave or particle description is best suited for understanding the experimental result. Reasoning that radiation and matter should be symmetrical in nature, Louis Victor de Broglie attributed a wave-like character to matter (material particles). The waves associated with the moving material particles are called matter waves or de Broglie waves. 9. The de Broglie wavelength ( ) associated with a moving particle is related to its momentum p as: = h/p. The dualism of matter is inherent in the de Broglie relation which contains a wave concept ( ) and a particle concept (p). The de Broglie wavelength is independent of the charge and nature of the material particle. It is significantly measurable (of the order of the atomic-planes spacing in crystals) only in case of sub-atomic particles like electrons, protons, etc. (due to smallness of their masses and hence, momenta). However, it is indeed very small, quite beyond measurement, in case of macroscopic objects, commonly encountered in everyday life. 287 Reprint 2026-27