Transcript Lecture on Light and Thermal Radiation

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 is it hot in my car?
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"[Of celestial bodies] We may determine their
forms, their distances, their sizes, and their motions
- but we can never know anything of their chemical
composition; and much less, that of organized
beings living on their surface.“
Philosopher Auguste Comte, 1835
Open Cluster NGC 290: A Stellar Jewel Box
Credit: ESA & NASA; Acknowledgement: E. Olszewski (U. Arizona)
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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. Take a star’s temperature.
2. Weigh a black hole.
3. Tell what our Galaxy looks like from the
outside.
4. See the beginning of the Universe.
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The “Visible” Spectrum
• When you think of “light”, what do you
think of?
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Light
• 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 of light is:
E = hn = hc/l
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E = hc/l
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Three Reasons
All objects do one or more:
1. Reflect light because of color or smoothness
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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Reflecting Light
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Temperature and
Light
• Warm objects emit light.
– Thermal radiation
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Kelvin Temperature
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•
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•
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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: 5800 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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Concept Test
Which of the following best
describes how Star A would
appear compared to Star B?
1. Star A would appear more red
than Star B.
2. Both stars would appear more red
than blue.
3. Both stars would appear more
blue than red.
4. Star A would appear more blue
than Star B.
5. None of the above.
Star A
visible
range
Star B
VIBGYOR
Wavelength
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Concept Test
Which of the two stars (A or B)
is at a higher temperature?
1.
2.
3.
4.
Star A
Star B
The two stars have the
same temperature.
It is not possible to infer
this relationship.
Star A
visible
range
Star B
VIBGYOR
Wavelength
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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.
E = hc/l
• But light IS energy.
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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
• 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.
(peak at shorter
wavelength)
2. Hotter is brighter.
(more intense at all
wavelengths)
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Graphically
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Graphically
lpeak = const/T
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Result
• If everything with a temperature gives off light,
why doesn’t we see everything glowing?
lpeak = k/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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Betelgeuse
The IR Universe
Orion - visible
• Everyday
things that
are hot
radiate in
the IR:
• Dust –
There are
interstellar
clouds of
dust.
Orion Nebula
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
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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