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Light and AtomsWhy study the behavior of
light and atoms?– It is only through light that
we know anything about the Universe.
– We can’t experiment on stars and planets.
– Light tells us about the position and velocity of a star or planet. It can also tell us about the temperature and the composition.
An infrared image of a man holding a match. White is the hottest temperature, Blue/Black are the coolest temperatures
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Light & Atoms
• Atoms interact with light by absorbing, emitting and bending light
• Atoms leave their unique signature or “fingerprint” in the light they emit or absorb – this can tell us what stars and planets are
made of– astronomers see these “fingerprints” in light
from objects in distant galaxies.
• Atoms in our own atmosphere can blur and absorb light from distant objects
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The Nature of Light• The speed of light (c) is
constant in a vacuum• Light has particle-like
properties (photons) and wave-like properties (wavelength and frequency)
• Which way you describe light depends on the kind of observation you are making.
Light is an electromagnetic wave
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The Nature of Light• Light has a wavelength and a
frequency – wavelength () - distance
between wave crests– frequency () - number of
wave crests that pass a point in 1 second
• When you tune your radio you are actually changing the frequency.
• A piano will emit different frequencies of sound based on which key you strike.
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Wavelength and Frequency
The wavelength () and the frequency () are related to the speed (c) by the formula
= cIf the wavelength increases the frequency must decrease for the speed to stay the
same
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Color and Frequency• The frequency and
color of light are related. Red light for example has a lower frequency than blue light.
• White light is a mixture of light of many different frequencies. A prism can break light into a rainbow (spectrum) of colors.
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Wave vs. Photon
• Wavelength/Frequency
– Short wavelength: blue
– Long wavelength: red
• Wave Amplitude
– Small amplitude
– Large amplitude
• Photon energy
– High energy
– Low energy
• Photon flux (num. photons/area)
– Low photon flux
– High photon flux
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Electromagnetic Radiation
• Visible light is just one form of electromagnetic radiation. • Together they form the electromagnetic spectrum. They differ in
their wavelength, frequency and energy.• However specifying the wavelength, frequency or energy uniquely
characterizes the form of electromagnetic radiation.
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The Energy of EM Radiation
The energy (E) carried by electromagnetic radiation is related to the frequency (). X-rays have a higher frequency and higher energy than radio waves for example.
T < 10 K
10 – 1,000 K
1,000 – 10,000 K
104 – 106 K
106 – 108 K
T > 108 K
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The Energy of EM Radiation
The energy (E) carried by electromagnetic radiation is related to the frequency (). X-rays have a higher frequency and higher energy than radio waves for example.
Cold gas / Accelerated electrons / Cosmic Background Radiation
Cool Stars / Warm dust Planets
Planets/Stars/Galaxies
Hot massive stars
Supernovae / Hot tenuous gas
Pulsars / Black Hole Accretion
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Multi-Wavelength Astronomy
• Different physical processes produce light at different wavelengths.
• To better understand a physical system (planet, star, galaxy), observe it in as many energy (wavelength) bands as possible.– Choose the bands to match the energy range for
the physical process of interest.
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Multi-Wavelength Astronomy
• The Sun
• Supernova Remnant Cas A
• Supernova Remnant M1, The Crab Nebula
• Giant Elliptical Galaxy M87
• Peculiar, Interacting Galaxy Centaurus A
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Wien’s Law• Wien’s law allows
astronomers to determine the temperature of a star.
• The wavelength at which a star is brightest is related to its temperature
• Hotter objects radiate more strongly at shorter wavelengths
• Blue has a shorter wavelength than red, so hotter objects look bluer.
•Objects can emit radiation at many different wavelengths.•The wavelength at which a star is brightest is related to its temperature. •This is Wien’s Law
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When can you use Wien’s Law?
• Only for objects that emit light not for those that reflect light
• Light emitted by hot, solid objects obey Wien’s Law
• Can not use with gases unless they are of a high density
• The Sun and other stars obey Wien’s Law since the gases they are composed of remain at a high density (at least up to the outermost layers of the star).