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Unformatted text preview: LIGHT Light
Light has wavelike characteristics.
Emitted light depends upon the object’s temperature.
Light has particlelike characteristics.
The motion of a light source affects wavelengths.
The t t
Th structure of atoms explains why each chemical element
f t
l i
h
h h i l l
t
emits and absorbs light at specific wavelengths. Wavelike Characteristics
Wavelength: Distance between successive crests (λ).
1 nm = 109 m
1 Å = 0.1 nm = 1010 m
Frequency:
F Number f
N b of crests th t pass a specific point in
t that
ifi
i ti
one second (ν or f). Speed: Frequency times Wavelength (c)
c = λ ν = 3 x 105 km/s Example
Yellow Light
ν = 6.0 x 1014 /s
c = νλ so λ = c/ν = (3 x 108 m/s) / (6.0 x 1014 /s) = 5 0 x 107 m
(6 0
5.0 Propagation of Light
Apparent Brightness
Flux = Luminosity / 4 π d2 Inverse Square Law
Flux
Fl 1
4 π d22
d22
 =  = Flux2
4 π d12
d12 Measuring the Speed of Light
Olaus Roemer (1675) timed
when moons disappeared in to
Jupiter s shadow.
Jupiter’s shadow
Eclipses were retarded when
y
Earth was far away.
Showed light did not travel
infinitely fast.
Did not know the EarthSun
id
k
h
hS
distance, so he could not
p
p
g
compute the speed of light. Measuring the Speed of Light
Jean Foucault (1850) used a
rapidly rotating mirror.
One
O must measure the
h
deflection angle and know the
p
g
speed of the rotating mirror.
Accuracy was better than 1%.
Modern Value
c = 299,792.458 km/s Visible Light
Visible or Optical Light
Range from 700 nm (Red) to
400 nm (Violet)
White light is a combination
of these colors. Newton’s
experiment proved that
p
p
prisms do not add color. Electromagnetic Spectrum
Wavelength Ranges
Gamma
Xrays
Ultraviolet
Visible
Infrared
Microwave
Radio wave 0.01
0 01 nm
0.01 to 20 nm
20 to 400 nm
400 to 700 nm
700 nm to mm
mm to cm
cm to km Example
FM Radio Station
ν = 94.9 MHz = 94.9 x 106 /s
c = νλ so λ = c/ν = (3 x 108 m/s) / (94.9 x 106 /s) = 3 2 m
(94 9
3.2 Light
Light has wavelike characteristics.
Emitted light depends upon the object’s temperature. Temperature Effects
Temperature determines the type of
electromagnetic radiation emitted.
Temperature is a measure of the average
motion of the gas molecules.
Electromagnetic radiation is emitted when
El
i di i i
i d h
electric charges accelerate – that is,
whenever they change either the speed or
the direction of their motion. Each
motion
collision results in the emission of
radiation. Color Indicates Temperature Temperature Scales
Kelvin Temperature Scale
Coldest Temperature = 0 K
(no atomic motion)
No negative temperatures
No degree symbol – just K
[The Mechanical Universe #45] Blackbodies
Consider an idealized object
that absorbs all the light that
impinges on it. Such an
it
object is called a blackbody.
A blackbody absorbs the
entire energy incident upon it
and heats up until it is
emitting energy at the same
g
gy
rate that it is being absorbed.
Are stars blackbodies? Blackbody Radiation
A blackbody with a temperature higher than absolute
zero emits some energy in all wavelengths.
A blackbody at higher temperature emits more
energy at all wavelengths than does a cooler one. Blackbody Curves Blackbody Radiation
A blackbody with a temperature higher than absolute
zero emits some energy in all wavelengths.
A blackbody at higher temperature emits more
energy at all wavelengths than does a cooler one.
The higher the temperature, the shorter the wavelength
at which the maximum energy is emitted.
emitted Blackbody Curves Wien’s Law
The wavelength of maximum energy is given by Wien’s Law
λ max = 0 0029 / T
0.0029
(where λmax is in meters and T is temperature)
Examples:
Sun λmax = 500 nm = 5.0 x 107 m T = 0.0029 / 5.0 x 107 = 5,800 K Rigel λmax = 240 nm = 2 4 x 107 m
2.4 T = 0 0029 / 2.4 x 107 = 12 000 K
0.0029 2 4
12,000 StefanBoltzmann Law
Total energy is provided by the StefanBoltzmann Law:
E = A σ T4
E = 4 π R2 σ T4
(σ is equal to 5.670 x 108 joule/m2) StefanBoltzmann Law
Example: E = 4 π R2 σ T4
ERigel / ESun = (RRigel / RSun)2 (TRigel / TSun)4
= (50 / 1)2 (12,000 / 5,800)4
= (2500) (18) = 45,000 X PRS Question
1. The Star Betelgeuse has a temperature of 3000 K and the Sun’s
temperature is about 6000 K. Assuming the Radii are the same,
what is the ratio of energies of Betelgeuse to the Sun?
a.
0.50
d.
16
b.
2.0
e.
0.06
c.
c
3000 Light
Light has wavelike characteristics.
Emitted light depends upon the object’s temperature.
Light has particlelike characteristics. Photoelectric Effect
Einstein (1905) analyzed the
Photoelectric Effect. He assumed
that light can be considered a
stream of photons. Each photon
has an energy equal to:
E = hν = hc/λ
h = 6.625 x 1034 J s Photoelectric Effect Example
1) λ = 500 nm = 500 x 109 m
E = h c / λ = (6.625 x 1034 Js)(3 x 108 m/s) / (500 x 109 m)
(
)(
) (
)
= 3.975 x 1019 J ( / 1.6 x 1019 J/eV) = 2.5 eV
2) λ = 0.2 nm = 0.2 x 109 m
(
)(
) (
)
E = h c / λ = (6.625 x 1034 Js)(3 x 108 m/s) / (0.2 x 109 m)
= 9.938 x 1019 J ( / 1.6 x 1019 J/eV) = 6211 eV Light
Light has wavelike characteristics.
Emitted light depends upon the object’s temperature.
Light has particlelike characteristics.
The motion of a light source affects wavelengths. Doppler Shift
If a light source is approaching or
receding from the observer, the
light waves will be, respectively,
crowded closer together or spread
out. The Doppler effect is only
produced by radial velocities.
Δλ/λ=v/c
v = c Δλ / λ PRS Question
2. The Star Betelgeuse is moving away from us at a speed of 300 km/s.
When one observes a spectral feature created at 500 nm, it will appear
to be shifted by how much?
a.
0.5 nm blueward
d. 5.0 nm blueward
b.
0.5 nm redward
e. 5.0 nm redward
c.
c
no shift is seen Light
Light has wavelike characteristics.
Emitted light depends upon the object’s temperature.
Light has particlelike characteristics.
The motion of a light source affects wavelengths.
The t t
Th structure of atoms explains why each chemical element
f t
l i
h
h h i l l
t
emits and absorbs light at specific wavelengths. Atomic Structure
What do spectral lines tell us about the structure of atoms?
After all, each element has its own unique set of spectral lines. Solar Spectrum Fraunhofer (1814) Shined sunlight through a prism and
highly magnified the spectrum. He saw 600 dark lines.
spectrum
lines Spectra of Different Elements ...
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 Spring '12
 Jarrio
 Light

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