Light and Atoms
Why 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
Light is an electromagnetic wave
• 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.
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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
=c
If 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
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colors.
Wave vs. Photon
• Wavelength/Frequency
– Short wavelength: blue
– Long wavelength: red
• Photon energy
– High energy
– Low energy
• Wave Amplitude
– Small amplitude
– Large amplitude
• 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
10 – 10 K
radiation
is
related
10 – 10 K
to the frequency ().
1,000 – 10,000 K
X-rays have a higher
10 – 1,000 K
frequency and
T < 10 K
higher energy than
radio waves for
example.
T > 108 K
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The Energy of EM Radiation
The energy (E) carried
by electromagnetic
Hot
radiation is related
massive
stars
to the frequency ().
Planets/Stars/Galaxies
X-rays have a higher
Cool Stars / Warm dust
frequency and
Planets
Cold gas / Accelerated
higher energy than
electrons / Cosmic
Background Radiation
radio waves for
example.
Pulsars / Black Hole
Accretion
Supernovae / Hot
tenuous gas
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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
•Objects can emit radiation at many
different wavelengths.
wavelength than red, so
•The wavelength at which a star is brightest hotter objects look bluer.
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).
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