Transcript Light and Temperature - University of Redlands

Light and Temperature
Astronomy: The Science of Seeing
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Goals
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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 lightyears away.
4. Tell what our Galaxy look like from the outside.
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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 a wave of energy.
• Moves 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
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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 = ln
• What does light look like?
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A Spectrum
Emission lines
Continuum
Absorption lines
• A spectrum = the amount of light given off by an
object at a range of wavelengths.
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Three Reasons
All 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
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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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Temperature and Light
• Warm objects emit light.
– Thermal radiation
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Kelvin Temperature
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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.
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(peak at shorter
wavelength)
2. Hotter is brighter.
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(More intense at
all wavelengths)
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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 = hc/l
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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.
HOT
COLD
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Energy and Intensity
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The more energetic the average collision the bluer the
average light that is given off.
– Since E = hc/l
• The more collisions that occur the more light that is
given off per surface area.
1. Hotter is bluer.
2. Hotter is brighter.
(peak at shorter
wavelength)
(more intense at all
wavelengths)
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Graphically
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 a 1/T
lpeak = k* 1/T
lpeak = (3 x 10-3 m/K) * 1/ 300 K
lpeak = 10-5 m
IR
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Thermal 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
Orion - visible
Orion – by IRAS
• Everyday
things that
are hot
radiate in the
IR:
• Dust – There
are
interstellar
clouds of
dust.
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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
The Moon in eclipse.
Orion – by IRAS
R. Gendler
• In eclipse, there is
no reflected light.
• Only thermal
radiation.
• Differences in
composition lead to
differences in
temperature.
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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
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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?
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Homework #4
• For Tuesday 1/27:
• Read B6.4 – 6.5
• Do B6: Problems 4, 8, 12, 13, 14, 17, 18
– (PHYS 170: replace problem 13, 17 with 22, 23)
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