A2/A-level Physics - Quantum Physics

Energy of a Photon

• Photon: a quantum of EM radiation
• E = hf
  • E = energy of a photon/J
  • h = Plank’s constant (6.63x10-34Js)
  • f = frequency/Hz

Experiment measuring Planck's Constant

1. Connect an LED of known wavelength in the electrical circuit shown.
2. Start off with no current flowing through the circuit, then adjust the variable resistor until a current begins to flow.
3. Record the voltage (V0) across the LED & the wavelength of light being emitted.
4. Repeat with LEDs of different wavelengths.
5. Plot a graph of V0 against 1/λ.
6. You should get a straight line with a gradient hc/e. Use this to find h.

Remember this equation, it is very useful in A2/A-level Physics exam!👨‍🏫

Wave-Particle Duality

• λ = h ÷ mv
• Electron diffraction shows the wave nature of electrons.
  • Electrons travelling through polycrystalline graphite will be diffracted by the atoms & spacing between the atoms.
  • Diffraction of electrons by matter can be used to determine arrangement of atoms and the size of the nuclei.

Energy Levels in Atoms

• Electrons in atoms exist in discrete energy levels.
  • Each level is given a number, n = 1 represents the ground state.
  • electrons can move down an energy level by emitting a photon.
  • The energy carried by each photon is equal to the difference in energies between two levels.
• ΔE = E2 – E1 = hf = hc/λ.

Hot gases produce line emission spectra

• As you heat gas, many of its electrons move to higher energy levels, and when they fall back to ground state, these electrons emit energy as photons.
• If you split the light from a hot gas with a prism, you get a spectrum.
• Each line on the spectrum corresponds with a particular wavelength.
• Since only certain photon energies are allowed, you only see the corresponding wavelength.

Spectrum

Reference:

https://getrevising.co.uk/resources/quantum_physics

This is the end of the topic!

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