Light and Temperature

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Light and Temperature

• Astronomy The Science of Seeing

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Goals

• 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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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 well see how we can use light to
• Weigh a planet.
• Take a stars temperature.
• Tell whats in the center of a star a thousand
  light-years away.
• 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

• 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
• Reflect light because of color or smoothness
  (same as scatter)
• Emit light because of their temperature
• (thermal radiation)
• 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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Temperature and Light

• Warm objects emit light.
• Thermal radiation

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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 5800 K

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Thermal Radiation Laws

• Hotter is bluer.
• (peak at shorter wavelength)
• Hotter is brighter.
• (More intense at all wavelengths)

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Concept Test

• Which of the two stars (A or B) is at a higher
  temperature?
• a. Star A
• b. Star B
• c. The two stars have the same temperature.
• d. It is not possible to infer this relationship.

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Concept Test

• Which of the following best describes how Star A
  would appear compared to Star B?
• a. Star A would appear more red than Star B.
• b. Both stars would appear more red than blue.
• c. Both stars would appear more blue than red.
• d. Star A would appear more blue than Star B.
• e. None of the above.

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Concept Test

• Which of the following best describes the light
  from Star A compared to Star B?
• a. More ultraviolet light but less visible
  light.
• b. More infrared light but less visible light.
• c. More visible light but less infrared light.
• d. Less infrared light and less ultraviolet
  light.
• e. More ultraviolet light and more visible
  light.

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Concept Test

• Imagine the Suns surface suddenly became much
  cooler (while the Suns size remains the same).
  Compared to the light it now emits, the Sun would
  emit
• More ultraviolet light but less visible light.
• More infrared light but less visible light.
• More visible light but less infrared light.
• Less infrared light and less ultraviolet light.
• More ultraviolet light and more visible light.

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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.

COOLER
HOTTER
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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

• 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

• 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

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Thermal versus Reflection

• Thermal radiation is light given off because of
  an objects temperature.
• Dont 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.

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The IR Universe

• Molten Rock There are lava flows on a moon of
  Jupiter.

Io from IRTF.
Orion by IRAS
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The IR Universe

• In eclipse, there is no reflected light.
• Only thermal radiation.
• Differences in composition lead to differences in
  temperature.

Orion by IRAS
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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 Whys

• 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 5

• For Monday 20 Sept Read B6.4 6.5
• Do B6 Problems 4, 14, 24
• Consider the four stars (A,B,C,D) of Problem 17
  in Chapter B6. Arrange them by speed relative to
  us (starting with the fastest moving towards us
  and ending with fastest away).
• A, B, C, D
• B, A, D, C
• D, C, B, A
• C, D, A, B
• None of the above