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G. Energy of a photon. You should be able to: describe the particulate nature (photon model) of electromagnetic radiation state that a photon is a quantum of energy of electromagnetic radiation select and use the equations for the energy of a photon: E = hf and E = hc / λ
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Energy of a photon You should be able to: • describe the particulate nature (photon model) of electromagnetic radiation • state that a photon is a quantum of energy of electromagnetic radiation • select and use the equations for the energy of a photon: E = hfand E = hc / λ • define and use the electronvolt (eV) as a unit of energy • use the transfer equation eV = ½ mv2 for electrons and other charged particles • describe an experiment using LEDs to estimate the Planck constant h using the equation eV = hc / λ
The Planck Constant h = 6.626 068 76 x 10-34 Js Energy (of a photon) is proportional to frequency E α f E = h f
The electronvolt 1V = 1JC-1 Qelectron = -1.60 x 10-19C 1eV = 1.60 x 10-19J
The electronvolt One electronvolt is the energy change of an electron when it moves through a potential difference of one volt. Its value is 1.60 x 10-19J
The photoelectric effect You should be able to: • describe and explain the phenomenon of the photoelectric effect • explain that the photoelectric effect provides evidence for a particulate nature of electromagnetic radiation while phenomena such as interference and diffraction provide evidence for a wave nature • define and use the terms work functionand threshold frequency • state that energy is conserved when a photon interacts with an electron • select, explain and use Einstein’s photoelectric equation hf = ϕ + KEmax • explain why the maximum kinetic energy of the electrons is independent of intensity and why the photoelectric current in a photocell circuit is proportional to intensity of the incident radiation.
The photoelectric effect Energy of a photon landing on the surface Kinetic energy of the electron Energy needed by an electron to escape the atom Imagine an electron is like a ball stuck in a deep hole. Energy is given to it by a light particle (a photon), if there is enough energy it can get out of the hole (or escape the atom) and any energy left over is the kinetic energy of the electron.
The photoelectric effect Electron does not have enough energy to escape the atom Energy of a photon landing on the surface If the photon does not have enough energy then the electron cannot escape the atom.
The photoelectric effect Energy of a photon landing on the surface. E = hf Kinetic energy of the electron. KEmax Energy needed by an electron to escape the atom. The work function, ϕ hf=ϕ+KEmax
The photoelectric effect hf=ϕ+KEmax hf = energy of a photon landing on the surface. Since ‘h’ is a constant, this is dependent of the frequency – if the frequency is too low then there may not be enough energy to release a photoelectron. ϕ = the work function. The amount of energy needed for an electron to escape, this is dependent on the type of metal (e.g it is lower for sodium than for zinc).
Threshold Frequency hf=ϕ+KEmax KEmax KEmax= hf–ϕ y = mx + c fo f -ϕ
Threshold Frequency The lowest frequency of radiation that will result in the emission of electrons from a particular metal surface.
Work function The minimum energy required to release an electron from a material.
We can vary the frequency of the incident light. photo current μA V p.d between plates
Wave-particle duality You should be able to: • explain electron diffraction as evidence for the wave nature of particles like electrons • explain that electrons travelling through polycrystalline graphite will be diffracted by the atoms and the spacing between the atoms • select and apply the de Broglie equation λ = h / mv • explain that the diffraction of electrons by matter can be used to determine the arrangement of atoms and the size of nuclei
Wave-particle duality This kind of pattern should be familiar to you. If we shine green monochromatic light through a diffraction grating we see an interference pattern caused by the constructive and destructive interference of waves. This proves the wave-like properties of light (but in this section we have been looking at its particle like properties) Light has wave-particle duality.
Wave-particle duality So if something we used to think of as a wave can behave like a particle, can something we think of as a particle behave like a wave? Yes! Electrons (the particles that orbit the nucleus) can also interfere with electrons to make a diffraction pattern.
Wave-particle duality Vacuum Electron gun Thin piece of polycrystalline graphite Electron diffraction pattern
De Broglie Equation A scientist by the name of De Broglie came up with the following equation linking the momentum of a particle with its wavelength: mv = h / λ Or λ = h / mv i.e. The faster the electron the shorter the wavelength, and the smaller the object it diffracts around (that is why scanning electron microscopes are used to look at very small objects – smaller than the wavelength of visible light)
