EMR 2.  When Hertz was testing Maxwell’s theory of electromagnetic waves he discovered that ultraviolet light caused some metallic surfaces to lose their.

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Presentation transcript:

EMR 2

 When Hertz was testing Maxwell’s theory of electromagnetic waves he discovered that ultraviolet light caused some metallic surfaces to lose their negative charge.  Explanation: When EMR strikes a metal surface, electrons are released from the plate.

 This is the photoelectric effect.  It refers to the observation that light shining on a negatively charged metal surface causes electrons to be emitted from that surface.

 The emissions are called photoelectrons.

 Not all types of EMR caused electrons to be emitted for a particular metal.  kIretM&feature=related kIretM&feature=related

 Photoelectrons are only emitted if the incident light is above a certain frequency, known as the threshold frequency (fo).  Different materials have different threshold frequencies.

 the emission of an electron or photoelectron from a substance when electromagnetic radiation falls onto it  Cathode Tube G e photoelectron metal photon - electrons which moved from the cathode to the anode produced a small current – indicated by the galvanometer.

 the minimum frequency of radiation needed to emit an electron from a given substance (the emission of the electron happens instantaneously; there is no build up over time)

 The release of electrons only occurs if a single photon hitting the metal target transfers its energy to one electron, giving that one electron the energy to move. The photon must have some minimum amount of energy to snap the electron off the metal. 3e The release of electrons only occurs if

 Is related to the energy needed to remove valence electrons from given atoms

 Increased frequency or decreased wavelength or increased energy of a photon results in increased E k of the emitted photoelectron.

 He applied Planck’s hypothesis of photons to describe this minimum amount of energy. He called it the work function of the metal, since you needed to do work on the electron to break it off.

 Each metal has a unique work function – chart on pg different metals hold on to their electrons with different strengths.

 Therefore, the work function is related to the threshold frequency. The formula for this is a modification of Planck's formula: E=hf W=hf o  W = work function (J)  h = Planck’s constant  f o = threshold frequency (Hz)

 Eg. 1 Determine the threshold frequency of a material with a work function of 9.6 eV.

photon e

 W=hf o Use this formula only when calculating things concerning electrons being knocked off of metal.  E=hf When just calculating individual photon energy, use Planck’s formula

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