Lecture6 sep23-bb (1)
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Transcript of Lecture6 sep23-bb (1)
1
Blackbody radiationBasic laws of radiation
Wien’s law max = 2897/T mmStefan-Boltzmann law: E = T4
Lecture 6
Solar and Earth Radiation
6,000 K 300 K
Earth is heated by visible radiation from the sun and cools by radiating infrared energy back to space
Solar Spectrum
The sun emits radiation at all wavelengths
Most of its energy is in the UV-VIS-IRportions of the spectrum
~50% of the energy is in the visible region ~40% in the near-IR ~10% in the UV
Radiation
Energy is transferred by electromagnetic waves
This includes all radiant energy:
X-rays Radio waves Light (sunlight) Microwaves
Properties of Waves
All electromagnetic waves travel at the same speedThe speed of light: 300,000 km/s
trough
crest
Properties of Waves
Wavelength
(length/cycle)
Wavelength (): the length of one complete cycle
trough
crest
Properties of Waves
Wavelength
(length/cycle)
Amplitude: 1/2 height between trough and crest
Amplitude
trough
crest
Properties of Waves
Wavelength
(length/cycle)
Frequency (): the number of cycles/second
Amplitude
trough
crest
Speed = wavelength x frequency
c =
(length/second) = (length/cycle) x (cycle/second)
Light speed is constantC= 300,000 km/s
Long wavelength (l), low frequency (n) short wavelength (l), high frequency (n)
Who is faster? Same
Whose frequency is higher? And why?
Energy of a wave
Energy is proportional to frequency,
and inversely proportional to wavelength
E = h
= h (c/ )
where h = Planck’s constant
In other words, waves with shorter wavelengths
(or higher frequency) have higher energy
Electromagnetic Spectrum
(m)
10001001010.10.01
denotes wavelength
m is micrometer unit of wavelength
110
103
10-310-210-1
10-6
10-9
1 meter (m)
1 kilometer (km)
1 cm (centimeter)1 mm (millimeter)
1 m (micrometer)
1 nm (nanometer)
103 meters = 1 kilometer
100 cm = 1 m103 mm = 1 m
106 m = 1 m
109 nm = 1 m
Re 0.65d m
0.51green m
0.42violet m
Different color light has different wavelength
Electromagnetic Spectrum
(m)
visiblelight
0.4 to 0.7 m
10001001010.10.01
ultraviolet infrared microwavesx-rays
LowEnergy
HighEnergy
( = “micro” = 10−6)
Radiant EnergyAll objects that have
temperature greater than 0K emit
radiation
sun and earth’s surface behave approximately as blackbodies
Basic laws of blackbody radiation
BlackbodyAny object that is a perfect emitter
and also a perfect absorber of radiation
Q: How much radiation is being emitted by an object, and at what wavelengths?
Q: At what wavelength does the earth emits most of its radiation? (Tearth=288K)
Q: At what wavelength does the sun emits most of its radiation? (Tsun=6000K)
Wien’s Law
Blackbody radiation curve
most objects emit radiation at many wavelengthshowever, there is one wavelength where an object emits
the largest amount of radiation energyThis wavelength is found with Wien’s Law
max
2897 m
(K)T
max
Q: At what wavelength does your body emits most of its radiation? (Tyou=310K)
Wavelength (m)
Incre
asin
g e
nerg
y
Wien’s law max = 2897/T mmThe Nobel Prize in Physics 1911
Rayleigh-Jeans Law
Bla
ckbody
Radia
tion
Wavelength (m)
Incre
asin
g e
nerg
y
Planck’s law
3
3
8 1( , )
1h
kT
hu T
ce
The Nobel Prize in Physics 1918
hPhotonenergy
Planck found that he could use a formula to fit the blackbody radiation curve
The beginning of quantum mechanics
Wavelength (m)
Incre
asin
g e
nerg
y hotter bodiesemit more andat higher energy(shorter wavelength)
E1=sT14
Total radiation energyStefan-Boltzmann law: E = T4
Boltzmann1844-1906 (Austria)
E2=sT24
Q: If T1=2T2, how many times E1 is E2?
His very famous formula (about entropy) is inscribed on Boltzmann's tombstone in Vienna.
Blackbody Radiation
Blackbody radiation—radiation emitted by a body that
emits (or absorbs) equally well at all wavelengths
Visible Light (VIS)
0.4 to 0.7 m
Our eyes are sensitive to this region of the spectrum
Violet-Indigo-Blue-Green-Yellow-Orange-Red
Infrared Radiation (IR)
We can’t see IR, but we can feel it as radiant heat
Lower energy than visible light
Ultraviolet Radiation (UV)
Higher energy than visible light
Can burn human skin and damage cells