Lesson 22: Blackbody Radiation!
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Part 1: The Blackbody Radiation Problem!
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Late 1800s !
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Evidence:!
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Problem:!
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All hot objects give off light.!
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Scientists found that hot glowing objects emit a continuous spectrum with
wavelength intensity peaking at a particular wavelength depending on the
temperature. !
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Continuous Spectrum: !
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Example: Human (37 Celsius)!
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Example: Glowing hot iron at (730 Celsius)!
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Example: The sun (Surface temp 4700 Celsius)!
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UV
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B
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R
IR
Black Body !
Intensity
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Scientists studied hot glowing objects
and measured the wavelengths and
intensity of the wavelengths emitted.
The results for temperatures ranging
from 3500 K to 5500 K are
summarized in the graph to the left. !
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The Blackbody Radiation Problem: !
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Part 2: Planck’s Solution to the Blackbody Radiation Problem!
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Max Planck was a German scientist who offered a solution to the
blackbody radiation problem at the beginning of the 20th century by
suggesting that light is quantized: !
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Planck’s quantization of light allowed him to develop a formula that very accurately predicted
blackbody radiation curve. !
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Planck’s Constant has two versions depending on the unit of energy you are using:!
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If you are working in Joules:!
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If you are working on Electron-Volts:!
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Review: Electron-Volt
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Example: How much energy does a 700 nm
photon have?!
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Example: A 100 W laser emits light at a frequency of 5x1014 hz. If it operates for 25 s, how many
photons are emitted?!
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Practice Problems:!
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10.
What is the energy (in Joules and eV) of a radio wave photon with a frequency of
100.7 MHz? (6.67 x 10-26 J, 4.17 x 10-7 eV )
What is the energy of a photon of blue light which has a wavelength of 450 nm?
(4.42 x 10-19 J)
What is the frequency of a quantum of EMR whose energy is 1.0 MeV? What type
of EMR is this? (2.4 x 1020 Hz)
A red neon laser has a wavelength of 632.8 nm. If the power of the laser is 1.50
mW, how many photons are being emitted per second? (4.77 x 1015)
A regular light bulb is about 5.0 % efficient in producing light energy from electrical
energy. How many photons are released per second by a 50 watt light bulb if the
average wavelength emitted is 500 nm? (6.3 x 1018)
When prints are developed in a darkroom, a red light bulb may be used for certain
types of film. Why does the red light not affect the photographic film?
What can you say about the relative temperatures of reddish, bluish, and whitishyellow stars?
If all objects radiate energy, why can’t we see them in the dark?
Describe what we mean by a “particle”; by a “wave.”
What are the differences between electrons and photons? Be specific.