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Light and Light and TemperatureTemperature

Astronomy: The Science of Seeing

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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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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 () 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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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: 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 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

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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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Graphically

peak 1/T F 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)?

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?

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Homework #4Homework #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)