4 light and temperature astronomy: the science of seeing
Post on 20-Dec-2015
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TRANSCRIPT
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GoalsGoals
• What is light?• What are the types of light?• Where does the light we see come
from?• Understanding the light of heat.• On a sunny day:
– Why does it seem hotter wearing a black T-shirt versus a white one?
– Why are they different?
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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).
• 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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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:
1. Weigh a planet.
2. Take a star’s temperature.
3. Tell what’s in the center of a star a thousand light-years away.
4. Tell what our Galaxy look like from the outside.
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What is Light?
• Light is a wave of energy.• Moves through a vacuum.• Travels at the speed of light (a
CONSTANT): c = 3 x 1010 cm/s
• The wavelength () and frequency () are related:
c =
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To Sum Up…To Sum Up…
• Radio waves, microwaves, rainbows, UV waves, x-rays, etc are ALL forms of light (electromagnetic waves).
• They ALL travel through space at the speed of light. c
• The higher the frequency, the shorter the wavelength. c =
• What does light look like?
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A Spectrum
• A spectrum = the amount of light given off by an object at a range of wavelengths.
Emission lines Absorption linesContinuum
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Three ReasonsAll objects do one or more:1. Reflect light because of color or smoothness
(same as scatter)2. Emit light because of their temperature
(thermal radiation)
3. Emit or absorb light because of their composition(spectral lines)
A person, house, or the Moon: reflects visible light, and because each is warm, emits infrared light.
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Reflection, absorption, and scatter
• Why do you see me?• Why do I see you?• Why is your shirt blue?• Why is this paper white?• Why is the table top black?
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Kelvin Temperature
• Kelvin: an absolute scale.• Kelvin is Celsius + 273 degrees.• Water freezes: 0 C 273 K• Water Boils: 100 C 373 K• Room Temp: 80 F 27 C 300 K• Surface Sun: 6000 K
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Thermal Radiation Laws
1. Hotter is bluer.– (peak at
shorter wavelength)
2. Hotter is brighter.– (More intense
at all wavelengths)
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Atoms and Light
• As atoms move they collide (interact, accelerate).
• Collisions give off energy.• But light IS energy.
E = hc
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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.
COLD
HOT
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Energy and Intensity
• The more energetic the average collision the bluer the average light that is given off.– Since E = hc
• The more collisions that occur the more light that is given off per surface area.
1. Hotter is bluer.(peak at shorter
wavelength)
2. Hotter is brighter.(more intense at all
wavelengths)
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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)?
peak 1/T
peak = k* 1/T
peak = (3 x 10-3 m/K) * 1/ 300 K
peak = 10-5 m IR
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Thermal versus ReflectionThermal versus Reflection
• Thermal radiation is light given off because of an object’s 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 thermal radiation do we see in our everyday life?
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The IR World• Everyday objects (at everyday
temperatures) emit thermal radiation in the IR, this is why we equate IR with 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 - visible
Orion – by IRAS
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The IR Universe
• Molten Rock – There are lava flows on a moon of Jupiter.
Orion – by IRAS
Io from IRTF.
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The Moon in eclipse.
The IR Universe
• In eclipse, there is no reflected light.
• Only thermal radiation.
• Differences in composition lead to differences in temperature.
Orion – by IRASR. Gendler
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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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Color Why’s
• Why is that shirt blue?• Why is the Sun yellow?• Why is this paper white?• Why is the light filament orange?• Why is Mars red?• On a sunny day:
– Why does it seem hotter wearing a black T-shirt versus a white one?