Transcript Light and Matter 1

Light and Matter
Astronomy: The Science of Seeing
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How do you do Astronomy?
• How do Chemists do Chemistry?
– Make solutions, mix chemicals …
• How do Biologists do Biology?
– Breed fruit flies, (and whatever else biologists do).
• They devise and conduct experiments in their labs.
• But how do you do that for astronomy?
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Light
• Astronomy is a “passive” science.
• We can’t (yet) go to the stars or other galaxies.
•The Universe must come to us.
•We rely on light exclusively!
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But what does it
look like from the
back?
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What you see is all you get!
So you need to squeeze EVERY last drop of
information out of the light we get.
This semester we’ll see how we can use light
to:
•Weigh a planet.
•Take a star’s temperature.
•Tell what’s in the center of a star a thousand
light-years away.
•Tell what our Galaxy look like from the outside.
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Tonight We’ll Learn:
• What is light?
• How is it produced?
– Continuum Blackbody radiation (this lecture)
– Line radiation (next lecture)
• How do light and matter affect each other?
• How are we able to learn about the Universe from
this light?
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The “Visible” Spectrum
• When you think of “light”, what do you
think of?
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What is Light?
• Light is an electromagnetic wave.
• Propagates through a vacuum.
• Travels at the speed of light (a CONSTANT):
c = 3 x 1010 cm/s
• The wavelength (l) and frequency (n) are related:
c = ln
• The energy is inversely proportional to the
wavelength (where h is a constant):
E = hn
E = hc/l
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Radio
g-ray
UV
Optical and infrared
X-ray
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What’s the Wavelength?
• Arrow 93.1 FM
• 93.1 MHz (Mega Hertz) = 93.1 x 106 cycles/sec
c = ln
3 x 1010 cm/sec = l x 93.1 x 106 cycles/sec
l = (3 x 1010 cm/sec)/(93.1 x 106 cycles/sec)
l = 322 cm =3.22 m
How big is your radio antenna?
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To Sum Up…
• Radio waves, microwaves, rainbows, UV waves,
x-rays, etc are ALL forms of electromagnetic
waves.
• They ALL travel through space at the speed of
light. c
• The higher the frequency, the shorter the
wavelength. c = ln
• The higher the frequency, the more energetic the
wave. E = hn
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The Sun
•Most objects emit light at
more than one wavelength,
thus producing a spectrum.
•Why?
•One Reason: Temperature!
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Matter
• Atoms consist of a positively charged nucleus
surrounded by a negatively charged cloud of one
or more electrons.
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Atoms in Motion
• Everything is composed of atoms which are
constantly in motion.
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Temperature
• The hotter the object, the faster the average
motion of the atoms.
HOTTER
COOLER
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Atoms and Light
• As atoms move they collide (interact, accelerate).
• Collisions give off energy.
• But light IS energy.
E = hn
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Light and Temperature
• The hotter the object the faster the average atom
and the more energetic the average collision.
• The faster the atoms the more collisions there are.
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HOT
COLD
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Energy and Intensity
• The more energetic the average collision the bluer the
average light that is given off.
– Since E = hn
• The more collisions that occur the more light that is
given off.
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Blackbody Laws
• Put another way:
Wien’s Law for peak wavelength (lpeak):
lpeak is proportional to 1/T
Stefan-Boltzmann Law for total Flux (F):
F is proportional to T4
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Graphically
Hottest
Hotter
Hot
lpeak a 1/T
F a T4
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Result
• HOT toasters are BRIGHTER than cool toasters.
• HOT toasters are BLUER than cool toasters.
• What is the peak wavelength for something at
room temperature (a cool toaster, or a cool
person)?
lpeak = k* 1/T
lpeak = (2.898 x 10-3 m/K) * 1/ 300 K
lpeak = 9.6 x 10-6 m
IR
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Blackbody Radiation
• Light given off by an object due solely to its
temperature.
• Don’t confuse with reflected light:
– Buses are yellow not because they are hot enough to
emit visible radiation but rather they reflect the yellow
light given off by the Sun.
• What kinds of blackbody radiation do we see in
our everyday life?
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The IR World
• Since everyday objects (at everyday temperatures) emit
blackbody radiation in the IR, this is why we perceive
IR as HEAT.
http://www.x20.org/library/thermal/blackbody.htm
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The IR Universe
• Everyday things
that are hot radiate
in the IR:
• Dust – There are
interstellar clouds
of dust.
Orion – by IRAS
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The IR Universe
Io from IRTF.
Orion – by IRAS
• Molten Rock –
There are lava
flows on a moon
of Jupiter.
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The IR Universe
• Differences in
composition lead to
differences in
temperature.
Orion
The –Moon
by IRAS
in eclipse.
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The Greenhouse Effect
• Why is my car hot on a summer day?
• At T = 6000 K, the Sun radiates mostly visible light.
Windshield is transparent to visible light.
• Car seat absorbs this visible light and warms up to
400 K.
• At T = 400 K, my seat radiates mostly at longer
wavelengths in the IR.
Windshield is opaque in the IR.
• Result: Energy is TRAPPED inside the car!
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Venus and Earth
• Certain gases act the same way as your
windshield: Carbon Dioxide (CO2).
• Venus – Runaway greenhouse effect.
• Earth – Could that happen here?
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