Dual Nature of Radiation and Matter
Class 12 Physics · Physics Part II · Chapter 3
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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.
Electron emission and work functionPhotoelectric effect and its lawsEinstein's photoelectric equationPhoton picture of lightde Broglie wavesDavisson-Germer experiment
Key terms
Work function
Minimum energy \(\phi_0\) needed to eject an electron from a metal surface.
Photoelectric effect
Emission of electrons from a metal when light above the threshold frequency falls on it.
Photon
Quantum of light energy \(E=h\nu\), behaving as a particle.
Threshold frequency
Minimum frequency below which no photoemission occurs.
de Broglie wavelength
Wavelength of a moving particle, \(\lambda=h/p\).
Stopping potential
Reverse voltage that just stops the most energetic photoelectrons.
Extra questions & answers
Try one from this chapter
The work function of a metal is the: