Transcript Chapter 3 Light and Atoms

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.
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.
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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?
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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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