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Physics AS - Nature of Light

AS Level EdExcel Physics (Unit 2)


on 2 June 2011

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Transcript of Physics AS - Nature of Light

Nature of Light The Photoelectric Effect Evidence of light having a 'particle' nature comes from the photoelectric effect. A photon is a packet of (electromagnetic) energy
When one photon is absorbed by a metal surface, its energy is transferred to a single electron, which may then be released from the metal This is called the photoelectric emission The electron released is known as a photoelectron The Work Function The work function is the minimum energy required to release the photoelectron from the metal surface. For photoelectric emmision to occur, the energy of the photon must be equal to or greater than the work function If the photon's energy, E, is just enough to release a photoelectron, then its frequency is called the threshold frequency Equations to remember: Where h is the Plank constant (6.63x10^-34Js) If the photon energy is less than the work function, the energy absorbed by the metal just causes a slight amount of heating The Wave Model & The Photon Model A wave model of light predicts that week radiation would eventually release large numbers of electrons with low energy - BUT this is not what happens... The Photoelectric effect can only be explained using a photon model.
Weak radiation causes release of some electrons IF the frequency is greater than the threshold frequency.
The kinetic energy of individual photoelectrons depends only on the frequency of the radiation - not intensity Stopping Voltage Is a potential: 1eV=1.6x10^-19J Photons, Spectra,
& Energy Levels Photon energy: light can be modelled as a wave OR as a photon. These equations link the two different models Spectra Emission Black background
Coloured lines Absorption Full spectrum background
Black lines - a particular frequency of light that is missing Each coloured line arises when an electron within an atom drops from one energy level to another and a photon is emmited Each dark line arises because an electon has absorbed energy and has moved from a lower to a higher energy level The photon energies can be calculated from mesurments of the wavelengths emitted or absorbed. Radiation Flux the rate at which a beam of light supplies energy to a particular area equation for calculating flux: Radiation Flux (Wm^-2) Power (W) Area (m^-2)
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