S117Lecture32011

S117Lecture32011 - Planck's quantum theory Let's look at...

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Planck’s quantum theory Let’s look at how the color of a filament changes as the temperature changes. As the resistive wire (filament) heats up it goes from red to white hot. Color is related to temperature! Black body radiation All objects at a given temperature emit radiation. (Think about the infrared pictures of a burglar sneaking up to the house that you have seen on late night TV). Using classical physics 19 th century scientists were unable to explain the relationship between color and temperature. They thought that the energy of light was related to its amplitude .
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Blackbody Radiation and the Ultraviolet catastrophe Sample at temp. T Cavity with non-reflecting walls Pinhole to observe emitted radiation Expect this classically Observe this! Classical physics predicts that the shortest wavelengths will dominate (there is no peak). In other words the universe would be filled with the highest frequency radiation (gamma rays) (life would be unable to exist).
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Planck’s proposal c h h E Max Planck proposed (c. 1900) that the oscillators in the sample could only vibrate at fixed frequencies. Namely, h = proportionality constant (now called Planck’s constant) = 6.626 x 10 -34 J.s h E Notice that given either wavelength or frequency we can calculate the energy or that given the energy both the wavelength and frequency are determined. Using this assumption Planck was able to explain blackbody radiation and “solve” the problem of the Ultraviolet catastrophe.
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Blackbody distributions A room temperature object An object near absolute zero
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More Blackbody distributions A “red hot” object ! But what is the physical meaning of h ?
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Photoelectric effect Experimental Observations: 1. Shine light on certain metals (e.g. alkali) and electrons are ejected. For example, shining red light on Cs causes the ejection of the electrons. Other metals require shorter wavelength (higher frequency) light. 2. For each metal there is a minimum frequency light necessary in order to cause ejection of the electrons. 3. Shining a lower frequency light on the metal, no matter how bright, does not result in the ejection of any electrons. 4. If you shine a higher frequency light on the metal, the kinetic energy of the electrons increased but the number of electrons remained the same. How could one understand these facts?
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Photoelectric effect: Experimental Setup
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Photoelectric effect: Dependence on intensity
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Photoelectric Effect: Dependence on frequency
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Photoelectric Effect: Work function and Planck’s constant  h KE electron
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Photons as particles 1865 picture of 1 inch Gatling gun
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This note was uploaded on 01/17/2012 for the course S 117 taught by Professor Stephenjacobson during the Fall '11 term at Indiana.

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S117Lecture32011 - Planck's quantum theory Let's look at...

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