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01:160:161 (178)
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Lecture 4

01:160:161 Lecture 4: Chem lecture 4 notes

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Rutgers University
M Rabeony

Photoelectric effect A phototube consists of an emitter coated with a metal which is capable of releasing electrons. Incident light shines on the emitter, and the emitted electrons are attracted to the collector, starting a photoelectric current, which is read on the meter. The energy of the light ionizes the metal, and the flow of electrons creates electricity. If you vary the frequency of the incident light, it is found that there is a certain minimum threshold frequency below which no electrons are emitted. This threshold frequency (v ) depends on the particular metal of the o emitter, but its independent of intensity. You could have a bright light (high intensity) but if you are below threshold frequency, no electrons are emitted. If you are above the threshold, the kinetic energy of the emitted electrons varies linearly with the frequency of the incident light. We perceive the light as a stream of particlelike units called photons. Each photon has a particular energy, dependent on its frequency. Photoelectric equation: hv=hv +oKE KE=hvhv o hv=energy of incoming photon hv = threshold energy, the energy needed to remove the electron from the metal o KE=energy of emitted electron A photon of energy hv strikes the emitter. This energy first must ionize the metal (remove the electron). If theres not enough energy to do that (vv ) then any remaining energy goes to the KE of o the emitted electron. Compare to y=mx+b slopeof the line =h (Plancks constant) H=6.6262x 10^34 J sec Each photon has an energy proportional to its frequency (E=hv). The intensity (brightness) of the light corresponds to the number of photons. A brighter light means more photons. However, the photons work individually to interact with electrons. One photon in: one electron out, providing that photon has sufficient energy (Hv>hv ) o Red is a safelight red is the lowest frequency and lowest energy photon of visible light. Red light photons isnt energetic enough to expose black and white photographic paper. Xrays are highenergy photons which are even more energetic than visible light. The energy of each photon is determined by the frequency of light: E=hv. The intensity (brightness) of the light is determined by the number of photons. Watt=unit of power=joulesec Energy of each photon=hv=hc Line Spectra of atoms
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