5 Light and Temperature Astronomy: The Science of Seeing

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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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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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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

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 two stars (A or B) is at a higher temperature?a. Star Ab. Star Bc. The two stars have the same temperature.d. It is not possible to infer this relationship.

V I B G Y O R

visible rangeStar A

Star B

Energy Output per second

Wavelength

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Concept TestWhich 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.

V I B G Y O R

visible rangeStar A

Star B

Energy Output per second

Wavelength

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Concept TestWhich 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. V I B G Y O R

visible rangeStar A

Star B

Energy Output per second

Wavelength

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Concept Test

• Imagine the Sun’s surface suddenly became much cooler (while the Sun’s size remains the same). Compared to the light it now emits, the Sun would emit: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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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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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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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 #5Homework #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. A, B, C, Db. B, A, D, Cc. D, C, B, Ad. C, D, A, Be. None of the above