CBSE Class 12 Board Examination

Chapter 3: Dual Nature of Radiation and Matter

Summary

Experiments on electron emission and the photoelectric effect revealed that radiation has a particle character, while matter shows wave behaviour—together establishing wave-particle duality. Electrons can be freed from a metal by heat, light, strong fields or collisions, the minimum energy needed being the work function \(\phi_0\). In the photoelectric effect, light above a threshold frequency ejects electrons instantly, the maximum kinetic energy depending on frequency, not intensity—facts classical wave theory cannot explain. Einstein resolved this with the photon idea: light is quantised into packets of energy \(E=h\nu\), and his photoelectric equation \(K_{max}=h\nu-\phi_0\) accounts for the threshold, the linear frequency dependence and the instantaneous emission. Intensity sets the number of photons, hence the current, while frequency sets each photon's energy. Conversely, de Broglie proposed that particles have an associated wavelength \(\lambda=\dfrac{h}{p}=\dfrac{h}{mv}\), confirmed by the Davisson-Germer experiment in which electrons diffract like waves. Thus radiation and matter each display both wave and particle aspects, a cornerstone of quantum physics.

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