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Quantum phenomena (Wave particle duality (• Light can behave either as…
Quantum phenomena
The photoelectric effect
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Einstein's explanation
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• His explanation: When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident ray and therefore gains energy = hf
• An electron can leave the metal surface is the energy gained from a single photon > the work function (φ) – minimum energy needed by electron to escape from metal surface
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Stopping potential (V)
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• At this potential the max kinetic energy of the emitted electron is reduced to 0 as each emitted electron must do extra work (equal to e × V) to leave the metal surface
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Energy levels in atoms
Electrons in atoms
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• When atom in ground state absorbs energy, one of its electrons moves to higher energy shell, so know in an excited state
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De-excitation
• Electron configuration in excited atom unstable because an electron which moves to outer shell leaves vacancy in previous shell
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• When this happens, the electron emits a photon so moves to a lower energy state - de-excitation
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Excitation using photons
• An electron in an atom can absorb a photon and move to a vacancy but only if the energy of the photon is exactly equal to the gain in electron’s energy
• Otherwise it won’t be absorbed by the electron
Fluorescence
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• This overall process explains why certain substances fluoresce (or glow) with visible light when the absorb UV
• Atoms in substance absorb UV photons and become excited, then de-excite and emit visible photons
• When source of UV removed, substance stops glowing
Fluorescent tube
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• When tube on, it emits visible light because ionisation and excitation of mercury atoms occur as they collide with each other and electrons in the tube
• The mercury atoms emit UV photons as well as visible photons and other photons when they de-excite
• UV photons absorbed by atoms of the fluorescent coating, causing excitation of atoms
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Wave particle duality
• Light can behave either as a particle or as a wave, depending on the circumstances
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• Particle like nature observed through photoelectric effect, or the fact that electrons in a beam can be deflected by a magnetic field
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• The wave-like behaviour of a matter particle is characterised by a wavelength – its de Broglie wavelength
• For de-Broglie wavelength, λ = h / (m × v), or, as m × v = momentum, λ = h / p
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