Dual Nature of Radiation and Matter
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Electron Emission and Threshold
Part of: Photoelectric Effect
Electrons escape metals by heat, light or strong fields, but light ejects them only when its frequency tops a threshold set by the work function, the minimum energy bill per electron. Below threshold no brightness helps, since each electron deals with one photon at a time.
Why it matters. Solar cells, cameras and night sensors all live at the threshold edge. Blaming brightness for what only frequency decides is the classic error.
Formulas
- Photon energy: E = h f = 1240 eV nm / lambda(nm)
- Threshold: h f0 = work function
- 1 eV = 1.6 x 10^-19 J
Key words
- Work function
- : Minimum energy needed to free an electron from a metal surface.
- Threshold frequency
- : Lowest light frequency that can eject electrons from a metal.
- Photoelectric emission
- : Electron ejection by light of sufficient frequency.
- Electron volt
- : Energy unit equal to 1.6 x 10^-19 joule.
Try it in two minutes
- Read the metal's work function in eV.
- Convert photon energy with E = 1240 eV nm over wavelength.
- Compare: eject only if photon beats work function.
Try at home. Explain why red light never charges a zinc plate but UV does.
Photoelectric Experiment Laws
Part of: Photoelectric Effect
Photocurrent grows with light intensity at fixed colour, while stopping potential grows with frequency and ignores intensity. The stopping voltage times electron charge equals the fastest electrons' energy, and the straight line against frequency has slope h over e.
Why it matters. Light meters and solar testers read these curves. Mixing up which knob changes current versus energy ruins every graph answer.
Formulas
- Max energy: Kmax = e x V0
- Photocurrent proportional to intensity at fixed frequency
- V0 line slope = h / e
Key words
- Photocurrent
- : Current from emitted electrons reaching the collector.
- Saturation current
- : Maximum photocurrent when all emitted electrons are collected.
- Stopping potential
- : Reverse voltage that halts even the fastest electrons.
Try it in two minutes
- Double intensity and watch current double.
- Raise frequency and watch stopping voltage climb.
- Multiply stopping volts by e for max energy.
Try at home. Sketch current versus voltage for two intensities and two colours.
Einstein Equation and Photons
Part of: Photons and Matter Waves
Einstein gave each photon energy hf: part pays the work function and the rest flies off as electron energy, so stopping voltage tracks frequency linearly. Photons carry momentum h over lambda despite zero mass, pushing comet tails and solar sails.
Why it matters. Photocells, lasers and quantum devices all budget photon energy this way. Forgetting to subtract the work function overstates electron energy.
Formulas
- Einstein: Kmax = h f - phi0
- Photon energy: E = h c / lambda
- Photon momentum: p = h / lambda
Key words
- Photon
- : Light packet with energy hf and momentum h over lambda.
- Einstein equation
- : Kmax equals photon energy minus work function.
- Planck constant
- : Quantum of action, 6.626 x 10^-34 J s.
Try it in two minutes
- Find photon energy from frequency or wavelength.
- Subtract the work function for max electron energy.
- Divide h by wavelength for photon momentum.
Try at home. Check that doubling wavelength halves photon energy.
Matter Waves and Electron Diffraction
Part of: Photons and Matter Waves
Every moving particle carries wavelength h over momentum, tiny for heavy bodies but nanometre-scale for electrons. Electrons accelerated through volts diffract like X-rays, proving matter waves, and higher voltage shortens the wavelength for sharper microscopes.
Why it matters. Electron microscopes beat light ones because of short electron waves. Calling wavelength charge-dependent, or expecting daily objects to diffract, misses the scale.
Formulas
- Matter wave: lambda = h / p
- Electrons: lambda(nm) = 1.227 / sqrt(V)
- Higher momentum means shorter wavelength
Key words
- de Broglie wavelength
- : Wavelength h over momentum carried by moving matter.
- Davisson-Germer experiment
- : Electron diffraction proof of matter waves.
Try it in two minutes
- Divide h by momentum for any particle.
- Use 1.227 over root volts in nm for electrons.
- Raise voltage to sharpen the wave.
Try at home. Compare wavelengths of an electron and a cricket ball at walking speed.
What your child will practise
- Einstein Equation and Photons: Ocean Quick Fire (Quick Fire)
- Electron Emission and Threshold: Desert Puzzle (Clue Puzzle)
- Electron Emission and Threshold: Jungle Quick Fire (Quick Fire)
- Matter Waves and Electron Diffraction: Candy Quick Fire (Quick Fire)
- Photoelectric Experiment Laws: Ocean Puzzle (Clue Puzzle)
- Quiz: Einstein Equation and Photons (8 questions)
- Quiz: Electron Emission and Threshold (8 questions)
- Quiz: Matter Waves and Electron Diffraction (8 questions)
- Quiz: Photoelectric Experiment Laws (8 questions)