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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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
• The energy of light is
• E hn hc/l

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The Visible Spectrum

• When you think of light, what do you think of?

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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
• The higher the frequency the more energy.
• E hn

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Concept Test

• Yellow light has a longer wavelength than blue
  light. Therefore yellow light has
• A higher frequency and higher energy than blue
  light.
• A higher energy and lower frequency than blue
  light.
• A lower frequency and lower energy than blue
  light.
• A lower energy and higher frequency than blue
  light.
• None of the above.

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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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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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Thermal Radiation Laws

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

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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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Color Whys

• Why is that shirt blue?
• Its hot.
• Its cool.
• Its hotter than a red shirt.
• Its reflecting blue light.
• Its absorbing blue light, thus making it blue.

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Color Whys

• Why is the Sun yellow?
• Its cooler than a blue flower.
• Its hotter than a person.
• Its reflecting yellow light.
• Its absorbing yellow light, thus making it
  yellow.
• None of the above.

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Color Whys

• Why is this paper white?
• Its temperature.
• Its hotter than the sun but cooler than a blue
  shirt.
• It reflects only white wavelengths of light, and
  absorbs the rest.
• It absorbs white light, thus making it white.
• It reflects all visible light equally.

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Color Whys

• Why is the light filament orange?
• It is hotter than a person, but cooler than the
  sun.
• It is cooler than a blue shirt.
• It reflects orange light.
• It absorbs orange light, thus making it orange.
• None of the above.

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Color Whys

• Why is Mars red?
• Its hot.
• Its cool.
• Its reflecting red light from the Sun.
• Its absorbing red light from the Sun.
• None of the above.

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Homework 8

• For Monday 22-Sept Read B5.3 5.5
• Do B5 Problems 35, 38,57
• Consider the four stars (A,B,C,D) of Problem 46
  in Chapter B5. 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
• Dont forget Topic of Confusion.