Chapter 3Light and Matter
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Units of Chapter 2Information from the Skies
Waves in What?
The Electromagnetic Spectrum
Thermal Radiation
Spectroscopy
The Formation of Spectral Lines
The Doppler Effect
Spectral-Line Analysis2
Information from the SkiesElectromagnetic Radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields
Example: Light
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Information from the Skies
~Light acts as both a wave and particle (photon)
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Light as a Wave
• Light waves are characterized by a wavelength, l,and a frequency, f.
f = c/l
c = 300,000 km/s = 3*108 m/s
• f and l are related through
l
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Information from the Skies
Wave motion: transmits energy without the physical transport of material
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Information from the Skies
Example: water wave
Water just moves up and down
Wave travels and can transmit energy
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Information from the Skies
Frequency: number of wave crests that pass a given point per second
Period: time between passage of successive crests
Relationship:
Period = 1 / Frequency
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Information from the Skies
Wavelength: distance between successive crests
Velocity: speed at which crests move
Relationship:
Velocity = Wavelength / Period
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Waves in What?
Diffraction: the bending of a wave around an obstacle
Interference: the sum of two waves; may be larger or smaller than the original waves
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Waves in What?
Diffraction: purely wave phenomenon
-If light were particles, there would be no fuzziness and light would be same size as the hole
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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?
What is the wave speed of electromagnetic waves?
c = 3.0 x 108 m/s
It can take light millions or even billions of years to travel astronomical distances
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Waves in What?
Magnetic and electric fields are inextricably intertwined.
A magnetic field, such as the Earth’s shown here, exerts a force on a moving charged particle.
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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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The Electromagnetic Spectrum
The visible spectrum is only a small part of the total electromagnetic
spectrum:
Different colors of light are distinguished by their frequency and wavelength.
Longest wavelength
Lowest frequency
Shortestwavelength
Highest frequency
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The Electromagnetic Spectrum
Different parts of the full electromagnetic spectrum have different names, but there is no limit on possible wavelengths.
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The Electromagnetic Spectrum
atmosphere is only transparent at a few wavelengths (opacity=thickness)
the visible, the near infrared, and the part of the radio spectrum with frequencies higher than the AM band
atmosphere absorbs a lot of the electromagnetic radiation impinging on it
astronomy at other wavelengths must be done above the atmosphere
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The Electromagnetic Spectrum
Need satellites to observe
Wavelength
Frequency
High flying air planes or satellites
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Thermal RadiationBlackbody Spectrum: radiation emitted by an object depending only on its temperature
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Thermal Radiation
• Temperature = amount of microscopic motion within
• Hotter = higher temp = faster particles =
MORE ENERGY• Intensity- amount/strength of radiation
at any point in space
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Thermal RadiationRadiation Laws
1. Peak wavelength is inversely proportional to temperature. Wien’s Law = λmax = 0.29 cm/T
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Thermal Radiation
~Blue Star is hotter than Red Star
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Figure 3.11
Thermal RadiationRadiation Laws
2. Total energy emitted is proportional to fourth power of temperature. Stefan’s Law
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More Precisely
Kelvin Temperature scale:
• All thermal motion ceases at 0 K
• Water freezes at 273 K and boils at 373 K
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Spectroscopy
Spectroscope: splits light into component colors
Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum.
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SpectroscopyEmission lines: single frequencies emitted by particular atoms
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SpectroscopyEmission spectrum can be used to identify elements:
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SpectroscopyAbsorption spectrum: if a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit
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SpectroscopyAbsorption spectrum of the Sun:
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Spectroscopy
Kirchhoff’s Laws:
• Continuous spectrum- Luminous solid, liquid, or dense gas produces
• Emission spectrum- Low-density hot gas
• Absorption spectrum- Continuous spectrum through a cool, thin gas
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Kirchhoff’s laws
Spectroscopy
Kirchhoff’s laws illustrated:
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Spectral Line AnalysisUsing spectrum, Astronomers can
determine:• Composition (atoms and molecules)• Temperature• Velocity• Rotation rate• Pressure of gas• Magnetic field
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The Doppler EffectIf one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer
Relationship between frequency and speed:
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Doppler Effect
• Redshift– Net motion away from observer– Shift toward longer wavelength
• Blueshift– Motion toward the observer– Shift toward shorter wavelength
Like train whistle: high pitch (towards) low pitch (away)
Also used for radar guns by police and in baseball
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The Doppler Effect
Depends only on the relative motion of source and observer:
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The Doppler EffectThe Doppler effect shifts an object’s entire spectrum either towards the red or towards the blue:
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Summary of Chapter• 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
• Can tell the temperature of an object by measuring its blackbody radiation
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Summary of Chapter
• Spectroscope splits light beam into component frequencies
• Continuous spectrum is emitted by solid, liquid, and dense gas
• Hot gas has characteristic emission spectrum
• Continuous spectrum incident on cool, thin gas gives characteristic absorption spectrum
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Summary of Chapter
• Spectra can be explained using atomic models
• Emission and absorption lines are distinct to different atoms and molecules
• Doppler effect can change perceived frequency of radiation
• Doppler effect depends on relative speed of source and observer
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Kirchhoff’s laws
Kirchhoff’s laws