Download How Matter Emits Light: 1. the Blackbody Radiation

How Matter Emits
Light:
1. the Blackbody
Radiation
Announcements
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Quiz # 3 will take place on Thursday, October
20th; more infos in the link `quizzes’ of the
website:
¨  Please,
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remember to bring a pencil.
Solutions for Exam # 1 are available from the
website, under `Exam
Looking ahead:
¨  Homework
# 3 is due on Thursday, Oct. 20th
¨  Homework # 4 starts on Thursday, Oct 20th . It is due
on Thursday, Oct. 27th
Assigned Reading
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Complete Unit 22;
Unit 23
Dimming with distance
n  As
you move away from a light source (a
light bulb, a street light, etc.) it becomes
dimmer. Why?
The energy emitted by
the source is constant,
but get spread over a
larger surface at larger
distance
Dimming with Distance
n  As
distance R increases, the area over
which the total light output L is distributed
increases as 4πR2
n  Thus:
L
______
Brightness =
4πR2
How Matter and Light Interact
Matter interacts with light in four
different ways:
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Absorption – the energy in the photon is
absorbed by the matter and turned into thermal
energy
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Reflection – no energy is transferred and the
photon bounces off in a new (and predictable)
direction
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E.g., Your hand feels warm in front of a fire.
E.g., Your bathroom mirror
Transmission – no energy is transferred and the
photon passes through the matter unchanged.
Emission – matter gives off light. Can be done in
two different ways, as we will see.
Absorption
Photon deposits energy into material. Thermal energy is increased
and the material gets warmer.
Transmission
Photon passes through material without depositing energy. Everything
remains unchanged.
Reflection
Photon reflects off of material. No energy is lost but outgoing photon
has a new direction.
These processes depend on both
the material and the wavelength of
the photon
Survey Question
Our eyes work via the process of:
1) absorption
2) reflection
3) transmission
4) emission
5) none of the above
Survey Question
Leaves are green because:
1) they only emit frequencies corresponding to
green
2) they only reflect frequencies corresponding to
green
3) they only transmit frequencies corresponding
to green
4) they only absorb frequencies corresponding
to green
The Difference Between Black and
White
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White light – contains all the frequencies
of the visible part of the spectrum.
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White paint – reflects all frequencies of the
visible part of the spectrum equally.
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Black paint – absorbs all frequencies of the
visible part of the spectrum equally.
Discussion Question
Why does NASA paint spacecraft white?
80%
Absorption
Absorption Spectrum
of Black Paint
40%
Visible
Infrared
Absorption Spectrum
of White Paint
0%
Frequency
Emission: How do objects make
light in the first place?
n  There
are two principal mechanisms for
producing electromagnetic radiation
¨ Blackbody
radiation
¨ Spectral line emission of atoms and
molecules
Both of these mechanisms result from
accelerating/decelerating electrons! I.e., you
accelerate or decelerate an electric charge to
create EM radiation
Light from Objects
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We perceive this `acceleration or `deceleration of
electrons as `light from objects
Imagine to heat up a piece of metal in a furnace:
¨  It will first turn red (temperature raising)
¨  then orange
¨  then yellow
¨  then whitish-blue (highest temperature)
The higher the temperature, the bluer the
object will appear
[you are linking temperature to color!]
What happens?
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The higher the temperature, the faster the atoms/
molecules in the object are (T ~ v2), thus more energetic
collisions
More energetic collisions cause more sudden
accelerations/decelerations of the electrons in the
matter, thus light with shorter wavelength (higher energy)
What does it mean?
n  Higher
Temperature = faster atoms
n  Faster atoms = more frequent and
energetic collisions
n  more frequent and energetic collisions =
more sudden electron accelerations/decel
n  more sudden electron accelerations/decel
= higher photon energy
n  Higher photon energy = bluer light
(E=hc/λ)
The color of light emitted is
connected to the Temperature
How to Measure Temperature
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Using the Kelvin Temperature scale:
¨  At T=0 K (the lowest possible temperature) all atoms/
molecules are still (virtually zero energy)
¨  Directly linked to an object s thermal energy (T=0 K
means zero thermal energy)
¨  Room temperature is 300 K
¨  Freezing water point 273.15 K, boiling point is 373.15 K
Celsius scale:
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Freezing water point = 0 C
Boiling water point = 100 C
Fahrenheit scale:
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Freezing water point = 32 F
Boiling water point = 212 F
Do not confuse Heat and
Temperature!
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Temperature refers to the
degree of motion of the
particles in a material, i.e.
the speed with which the
particles move (T~kinetic
energy~v2).
Heat refers to the amount of
energy stored in a body as
motion among its particles
and depends on density as
well as temperature.
Survey Question:
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You heat an oven to 450 F, and you also, separately,
boil some water in a pot (boiling point is 212 F). What
happens if you stick your hand first in the oven and then
in the boiling water [don t do that!]? Why?
I get burned in both
I only get burned in the oven, because of the higher
temperature
I only get burned in the water, because of the higher
heat
Survey Question:
n 
You heat an oven to 450 F, and you also, separately,
boil some water in a pot (boiling point is 212 F). What
happens if you stick your hand first in the oven and then
in the boiling water [don t do that!]? Why?
I get burned in both
I only get burned in the oven, because of the higher
temperature
I only get burned in the water, because of the higher
heat
What is a Blackbody?
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An object that can absorb all the radiation falling on it
(light at all wavelengths), so it appears black when cold
When it gets heated up, it can also emit radiation at all
wavelengths (think of the heated piece of metal).
A stove burner (conducting material), a furnace, planets
(radiating solids), and stars (dense gas) are excellent
examples of close-to-blackbodies
Materials that are insulating, non-burning, or liquid are
usually not good blackbodies
Blackbody Radiation
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Take a blackbody (e.g., a piece of metal)
and heat it up.
After it becomes hot, keep the temperature
constant (this is called thermal equilibrium)
Then plot, on a graph, the intensity of the
radiation (light) emitted as a function of
wavelength: this is called a spectrum
The shape of the spectrum and the maximum intensity of a
Blackbody will only depend on the Temperature
Think of Temperature as motion of the atoms/molecules in the
blackbody; if T=constant, the motion does not change, and the
acceleration/deceleration of the electrons also does not change. The
`color of the B.B. will not change.
Black Body radiation is the e.m. emission of matter at
thermal equilibrium (constant T)
One Temperature=one spectrum
Blackbodies are excellent thermometers
Wien’s Law
n  Hotter
objects emit photons with a
higher average energy = shorter
wavelength.
¨  The
peak of the blackbody emission spectrum is
given by
6
"max
2.9 #10
=
nm
T(Kelvin)
Stefan-Boltzmann Law:
n  Hotter
objects emit more total radiation per
unit surface area.
n  The luminosity of a hot body rises rapidly
with Temperature: L=A σT4
Survey Question
The graph below shows the blackbody spectra of three different
otherwise identical stars. Which of the stars is at the highest
temperature?
1) Star A
2) Star B
3) Star C
Relative
Intensity
A
B
C
Wavelength
Emitted power = σ A T4
Survey Question:
You are gradually heating two rocks (one larger than the
other) in an oven to an extremely high temperature. As
they heat up, the rocks emits nearly perfect theoretical
blackbody radiation – meaning that
1) the larger rock is bluer and brighter.
2) the larger rock is redder and brighter.
3) the larger rock is bluer but the same brightness.
4) the larger rock is the same color but brighter.
5) the larger rock is the same color and brightness.
Emitted power = σ A T4
Survey Question:
You are gradually heating two rocks (one larger than the
other) in an oven to an extremely high temperature. As
they heat up, the rocks emits nearly perfect theoretical
blackbody radiation – meaning that
1) the larger rock is bluer and brighter.
2) the larger rock is redder and brighter.
3) the larger rock is bluer but the same brightness.
4) the larger rock is the same color but brighter.
5) the larger rock is the same color and brightness.
Survey Question
The graph below shows the blackbody spectra of two totally
different stars. What can you conclude from the plot about
the two stars?
1) Star A is hotter but smaller than Star B
2) Star A is hotter and larger than Star B
3) Star A is cooler and larger than Star B
4) Star A is cooler and smaller than Star B
B
A
Relative
Intensity
Wavelength
Summary
n  Blackbody
Radiation (a.k.a. Thermal
Radiation)
¨ Many
objects with a temperature greater than
absolute zero (0 K) emit blackbody radiation.
¨ Hotter objects emit more total radiation per
unit surface area.
¨ Hotter objects emit photons with a higher
average energy.
Survey Question
n  You
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emit radiation:
True
False
Survey Question
n  You
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¨ 
emit radiation:
True
False
Your skin feels warm, you emit infrared
radiation