Class 12 Physics Dual Nature of Radiation and Matter Notes

11.1 Introduction

Light and matter exhibit both wave-like and particle-like properties. This concept is called the dual nature of radiation and matter, forming a cornerstone of quantum physics.

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11.2 Electron Emission

• Electrons can be emitted from metal surfaces by heating, light, or electric fields.
• Types of electron emission:
  • Thermionic emission – emission by heating
  • Photoelectric emission – emission by light
  • Field emission – emission by strong electric field

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11.3 Photoelectric Effect

• Emission of electrons from a metal surface when light of sufficient frequency shines on it.
• Key observations:
  • Electrons are emitted instantaneously.
  • Emission occurs only if light frequency exceeds threshold frequency (f0f_0f0​).
  • Maximum kinetic energy of electrons depends on light frequency, not intensity.

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11.4 Experimental Study of Photoelectric Effect

• Set up includes a photoelectric cell, voltmeter, and light source.
• Measure stopping potential to determine maximum kinetic energy of emitted electrons.
• Confirms instantaneous emission and frequency dependence of electron energy.

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11.5 Photoelectric Effect and Wave Theory of Light

• Wave theory predicts energy depends on light intensity, not frequency.
• Observations contradict wave theory, showing the need for a particle explanation of light.

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11.6 Einstein’s Photoelectric Equation: Energy Quantum of Radiation

• Einstein proposed light is quantized into photons.
• Energy of photon:

$$E = h f$$E=hf

• Photoelectric equation:

$$K_{\text{max}} = h f – \phi$$Kmax​=hf−ϕ

• $K_{\text{max}}$Kmax​ = maximum kinetic energy of electrons
• $h$h = Planck’s constant
• $f$f = frequency of incident light
• $\phi$ϕ = work function of the metal

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11.7 Particle Nature of Light: The Photon

• Photon: Quantum of electromagnetic radiation with energy $E = hf$E=hf.
• Explains photoelectric effect, Compton scattering, and blackbody radiation.
• Photon has zero rest mass but carries momentum p=E/cp = E/cp=E/c.

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11.8 Wave Nature of Matter

• de Broglie Hypothesis: Particles of matter, like electrons, have wave properties.
• Wavelength of particle:

$$\lambda = \frac{h}{p} = \frac{h}{mv}$$λ=ph​=mvh​

• Verified by electron diffraction experiments.
• Forms the basis of quantum mechanics and electron microscopes.