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Waves and Properties of Light
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Introduction
Information from the Skies
Waves in What?
The Wave Nature of Radiation
The Electromagnetic Spectrum
Thermal Radiation
The Kelvin Temperature Scale
More about the Radiation Laws
The Doppler Effect
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Information from the Skies
Electromagnetic Radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields
Example: Light
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Wave Properties
Wave motion: transmits energy without the physical transport of material
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Wave PropertiesExample: water wave
Water just moves up and down
Wave travels and can transmit energy
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Wave PropertiesFrequency: number of wave crests that pass a given point per second
Period: time between passage of successive crests
Relationship:
Frequency = 1 / Period
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Wave PropertiesWavelength: distance between successive crests
Velocity: speed at which crests move
Relationship:
Velocity = Wavelength / Period
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Light
Visible spectrum:
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Waves in What?
Water waves, sound waves, and so on, travel in a medium (water, air, …)
Electromagnetic waves need no medium
Created by accelerating charged particles:
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Waves in What?Electromagnetic waves: Oscillating electric and magnetic fields. Changing electric field creates magnetic field, and vice versa
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Waves in What?
What is the wave speed of electromagnetic waves?
c = 3.0 × 108 m/s
This speed is very large, but still finite; it can take light millions or even billions of years to traverse astronomical distances
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Waves in What?
The wave nature of radiation: radiation diffracts, which is purely a wave phenomenon
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The Electromagnetic Spectrum
No limit on wavelengths
Different ranges have different names
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The Ultraviolet Catastrophe
• Recall that shorter waves correspond to higher energy.
• As objects get hotter and hotter they should emit more and more energy at higher and higher frequencies or energies.
• This was not observed in the spectrum.
• Instead a sharp drop in intensity is observed!!
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Thermal Radiation
Blackbody Spectrum: radiation emitted by an object depending only on its temperature
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Thermal RadiationTemperature is a measure of the velocity of high speed molecules impacting the thermometer bulb.
Kelvin Temperature scale:
• All thermal motion ceases at 0 K
• Water freezes at 273 K and boils at 373 K
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Thermal RadiationRadiation Laws
Wien’s Law
1.Peak wavelength is inversely proportional to temperature,
p 1/T.
or
Peak frequency is proportional to the temperature,
fp T.
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Thermal Radiation
Radiation Laws
Stefan-Boltzmann Law
2. Total energy emitted is proportional to fourth power of temperature (note intensity of curves)
E T4
E is the energy emitted per unit of area.
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NOS and Scientific Laws
• Laws are descriptions of phenomena such as the two radiation laws previously discussed.These are simply observed to happen in a particular way such as E T4.
• Theories are models that explain why phenomena occur, or why laws are obeyed.
• So E T4 is the Stefen-Bolzmann Law, which is explained by Kinetic Theory and Quantum Mechanics that relates the energy of moving particles, like molecules, to the temperature of an object.
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Star Color and Temperature• We observe stellar colors and get their
temperatures from Wien’s Law. Red stars are cooler than blue stars.
• We can then estimate their energy output using Stefen-Boltzmann Law.
• In the picture to the left which star is cooler?
• Which star puts out more energy per unit area?
• Which star puts out more total energy?
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Doppler Effect
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The Doppler Effect
If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer
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The Doppler Effect
Depends only on the relative motion of source and observer:
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The Doppler Effect
Relationship between frequency and speed:
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Summary
• Wave: period, wavelength, amplitude
• Electromagnetic waves created by accelerating charges
• Visible spectrum is different wavelengths of light
• Entire electromagnetic spectrum:
radio waves, infrared, visible light, ultraviolet, X rays, gamma rays
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• Can tell the temperature of an object by measuring its blackbody radiation
• Doppler effect can change perceived frequency of radiation
• Doppler effect depends on relative speed of source and observer
Summary, cont.
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Resources
Chaisson & McMillan, (2002, 2004). Astronomy Today (4th & 5th Ed.)
Bennett et al. (2004) The Cosmic Perspective (3rd Ed.)
Shipman, Wilson, and Todd, (2003). AnIntroduction to Physical Science (10th
Edition).