Transcript Light and Temperature - University of Redlands

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 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 (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
Emission lines
Continuum
Absorption lines
• 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:
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
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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.
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(peak at shorter
wavelength)
2. Hotter is brighter.
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(More intense at
all wavelengths)
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Concept Test
Which of the two stars (A or B) is
at a higher temperature?
Star A
visible
range
a. Star A
b. Star B
c. The two stars have the same
temperature.
d. It is not possible to infer this
relationship.
Star B
VIBGYOR
Wavelength
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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.
Star A
visible
range
Star B
VIBGYOR
Wavelength
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Concept Test
Which of the following best
describes the light from Star A
compared to Star B?
Star A
visible
range
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.
Star B
VIBGYOR
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/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
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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.
2. Hotter is brighter.
(peak at shorter
wavelength)
(more intense at all
wavelengths)
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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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The IR Universe
Orion - visible
Orion – by IRAS
• Everyday
things that
are hot
radiate in the
IR:
• Dust – There
are
interstellar
clouds of
dust.
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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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Color Why’s
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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
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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.
e.
A, B, C, D
B, A, D, C
D, C, B, A
C, D, A, B
None of the above
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