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Any questions about the satellite assignment? Problems : Q3B.1, Q3B.2 and Q3B.5 due Wednesday If there are any errors on your printout circle them and return the printout to me.

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Chapter Q3 The Particle Nature of Light Problems due Wednesday Q3B.1, Q3B.2 and Q3B.5

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Is light a wave or a particle? The fact that light undergoes interference and diffraction proves that light is a wave. There are several situations in which light acts as a particle. One of these is when light shines on certain metals. –Electrons are given off when light hits these metals –The way in which the electrons are given off shows that light does not always act like a wave but is sometimes a particle.

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The Photoelectric Effect The release of electrons by a metal when a light shines on it has many important applications. –Digital cameras –Video cameras –Photocells – Cells used to measure the intensity (brightness) of light

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The photocell A Conductor (anode) (positive) Metallic plate (cathode) (positive) Ammeter light When light is shone on certain metals, electrons are given off (This is called the photoelectric effect.) These are enclosed in a glass bulb with a vacuum inside. This is an example of a photocell

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The photocell A Conductor (anode) (negative) Metallic plate (cathode) (positive) Ammeter light When light is shone on certain metals, electrons are given off (This is called the photoelectric effect.) Two types of experiments are done 1) The intensity and wave length are changed and the current is measured. (Cathode negative and anode positive) 2) A negative charge (voltage) is placed on the anode and the current as a function as a function of that voltage is measured. These are enclosed in a glass bulb with a vacuum inside.

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What determines whether or not an electron can get to a surface with a negative voltage V? e-e- ------------------------ Kinetic energy = electrical potential energy ------------------------

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The electron-volt A common unit of energy in atomic physics is the electron-volt. This is the energy an electron will have if it is accelerated through a potential of one volt. One electron volt = 1.6 x 10 -19 columbs 1 volt = 1.6 x 10 -19 joules.

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What wave theory predicts When an electromagnetic wave strikes an area the energy of the wave is spread over the whole area the light hits. –This means it takes time for a single electron to build up the energy necessary to free itself from the metal. At least several seconds are necessary before the electron will have the energy necessary to escape the metal. –The higher the intensity of the light, the more kinetic energy the electrons given off will possess.

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What really happens The first electrons are given off just as soon as the first light strikes the surface. The maximum kinetic energy of the electrons given off does not depend on the intensity of the light. The kinetic energy of the electrons that are emitted depends on the wavelength of the light. –The shorter the wavelength, the more K.E. the electrons have.

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What really happens If the wavelength is longer than a certain value (frequency lower) no electrons are given off, regardless of the intensity If the wavelength (frequency) is held constant the maximum K.E. is constant regardless of the intensity. –A brighter light gives off more electrons, not electrons of a higher energy Above the cutoff frequency, the slope of the K.E. vs. frequency curve is the same for all metals.

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The maximum kinetic energy of the photoelectrons given off when light strikes a metal is not determined by the intensity of the light, but by the frequency (wavelength) of the light. The shorter the wavelength (higher the frequency) the greater the kinetic energy. Different metals require the electron to have more or less energy before it can escape the surface of that particular metal. This amount of energy is called the “work function” and the symbol is “W”.

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Einstein again! In 1905 Einstein wrote three famous physics papers –Relativity –Brownian Motion Explaining why very small particles such as smoke in air or water continually jump around –The photon theory of light – light acts as a particle! Light travels in packets (called photons), the energy of one photon is given by E=hf (h is a constant called Plank’s constant. h = 6.626 x 10 -34 joule·seconds.

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This shows the entire electromagnetic spectrum, frequency (ν) and wavelength (λ) Examples: Compare the energy of a photon of light (600nm) and a photon of uv(300 nm) radiation.

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E=3.3 x 10 -19 j E=2.1 eV Case 1: Visible light E=6.6 x 10 -19 j E=4.2 eV Case 2: Ultraviolet radiation

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Equations and examples The maximum kinetic energy of a photoelectron given off when light of wavelength λ strikes a metal with a work function W, is given by. The work function W is the energy the electron uses in escaping from the surface of the metal.

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A 100 watt light bulb gives off 10 watts of light and 90 watts of heat. If we consider all the light to be of wavelength 600nm, how many photons/sec will strike a 18cm x 24 cm page that is 2 m from the bulb. The energy of one photon Total number of photons emitted by the light in a second (10 w) Total number of photons that hit the page that is 2 m from the light is:

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The work function of aluminum is 4.08 eV. What is the maximum wavelength of light that will eject electrons from aluminum? The lowest energy radiation that will eject electrons will be when K=0 λ=3.05 x 10 -7 m=305nm

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The Electromagnetic Spectrum Long Wavelengths Short Wavelength Low EnergyHigh Energy

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Problems (due Monday) Q3B.1, Q3B.2 and Q3B.5

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Particle Nature of Light

Particle Nature of Light

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