Why did the photoelectric effect disprove classical wave theory?

Classical wave theory predicted: (1) Any frequency light should eject electrons (given enough intensity) — experimentally false; (2) Higher intensity should give electrons more energy — experimentally false; (3) There should be a time lag for energy accumulation — experimentally false (emission is instantaneous). Einstein's quantum explanation (photons) explained all observations correctly.

What is work function and how does it vary for different metals?

Work function (φ) is the minimum energy required to eject an electron from the surface of a metal. It varies by metal: Cesium (1.9 eV) — easiest to eject (used in phototubes); Potassium (2.3 eV); Sodium (2.36 eV); Platinum (6.35 eV) — hardest. Higher work function metals need higher frequency light for photoelectric effect.

What is de Broglie's hypothesis?

De Broglie (1924) hypothesised that matter, like light, has a dual nature — it can behave as both a particle and a wave. The wavelength λ = h/mv. Implications: electrons in atoms form standing waves (explains Bohr's quantisation). Confirmed by Davisson-Germer experiment (1927). Led to development of electron microscopes.

Photoelectric Effect

Dual Nature of Radiation and Matter • Class 12 Physics • NCERT • CBSE

Einstein's Photoelectric Equation: KE_max = hν − φ = h(ν − ν₀). Maximum kinetic energy is independent of intensity; depends only on frequency. Stopping potential V₀: eV₀ = KE_max = h(ν − ν₀). de Broglie wavelength λ = h/p = h/(mv).

Key Formulas

KE_max = hν − φ
eV₀ = hν − φ (stopping potential)
φ = hν₀ (work function)
λ = h/mv (de Broglie wavelength)
λ = 1.226/√V nm (electrons)

Frequently Asked Questions

Why did the photoelectric effect disprove classical wave theory?: Classical wave theory predicted: (1) Any frequency light should eject electrons (given enough intensity) — experimentally false; (2) Higher intensity should give electrons more energy — experimentally false; (3) There should be a time lag for energy accumulation — experimentally false (emission is instantaneous). Einstein's quantum explanation (photons) explained all observations correctly.
What is work function and how does it vary for different metals?: Work function (φ) is the minimum energy required to eject an electron from the surface of a metal. It varies by metal: Cesium (1.9 eV) — easiest to eject (used in phototubes); Potassium (2.3 eV); Sodium (2.36 eV); Platinum (6.35 eV) — hardest. Higher work function metals need higher frequency light for photoelectric effect.
What is de Broglie's hypothesis?: De Broglie (1924) hypothesised that matter, like light, has a dual nature — it can behave as both a particle and a wave. The wavelength λ = h/mv. Implications: electrons in atoms form standing waves (explains Bohr's quantisation). Confirmed by Davisson-Germer experiment (1927). Led to development of electron microscopes.

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