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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).