Light

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Light the Electromagnetic Spectrum
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Messenger from the Universe

• Understand lights nature
• Learn its language what it can tell us about
  its source.

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(1) Speed of Light
Light travels damn fast.
Speed first measured by Ole Roemer (1675)
Modern value c 300,000 km s-1
3 x 108 m s-1
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(2) Nature of Color
Newton used a prism to find that white
light contains a rainbow of colors (ROYGBV)
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(3) Light as waves
a) Huygens argued light is a wave Waves
spread out diffraction
Incoming waves
Blurring spread
Newton thought light was a particle
corpuscular
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Wavelength, ?
?
b) Thomas Young measured its wavelength
using two slit interference pattern
?
Multiple fringes
c) Lights wavelength is very small red ?
blue
400 ? 700 nm 4000 ? 7000 Å
roughly cell-sized
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(4) Waves of what?
Maxwell (1860) studied electric magnetic fields
Derived four equations
Predicted E-M waves of Velocity c300,000 km s-1
? Light is an EM wave !!
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(5) Other wavelengths
Over the next 100 years, e-m waves of different
? discovered
radio
Astronomers now use the entire electromagnetic
spectrum here are some examples of non-visual
images ?
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Radio telescope
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Infrared Ultraviolet X-ray Telescopes
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(6) Atmospheric Windows
Parts of the em-spectrum are blocked by our
atmosphere
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(7) Frequency the wave equation
Frequency, f number of waves passing any point
per second.
l
c 300,000 km/s 3108 m/s
c f l
Since f waves each of length ? pass in 1 sec, we
have
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Em-waves often have very large frequencies ?
The wave equation also reads f c/? ? recall
c is huge and ? is tiny ? f is gynormous
Example what is frequency of green light, ?
500 nm f 3 x 108 / 500 x 10-9 6 x
1014 Hz 6 hundred thousand billion
oscillations per second
Notice the units of f are Hz (Hertz) per
sec (s-1) Recall for radio, one often
encounters MHz or GHz
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f and l are equivalent measures for an em-wave
wavelength
radio
1014
1016
1018
1020
1012
108
106
1010
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(8) Light as particles photons
Einstein (1905) explained the photoelectric
effect Photons of light have energy proportional
to frequency
E h f hc / ?
h Plancks constant 6.6 x 10-34 J s (Joules
x seconds)
Photon energies are very small e.g. red light
f5 x 1014 Hz Ered 6.6 x 10-34 x 5 x 1014 3 x
10-19 J Flashlight of 30 Watt ( 30 J/s) emits
30/3x10-19 1020 photons/s
100 billion billion per sec.
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(9) Waves and/or particles?
Is light a wave or a particle ? It has properties
of both one might say it is a wav-icle
A wave packet localized bundle of waves
Particles also behave like waves e.g. protons
electrons will diffract and interfere
In modern (quantum) physics, everything behaves
this way. (more massive things are more
particle-like eg a football)
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(10) Creation of Light
Two basic ways light is created
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(11) Thermal energy temperature
What does it mean to say something is hot
Molecules/atoms are in constant jostling motion
qualitatively hotter ? faster motion
quantitatively T proportional to
(average kinetic energy per particle)
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(12) Thermal Radiation
Bouncing molecules ? photons Spread of energies ?
range of colors Higher energy bounce ? bluer
photons
Pure creation of light also means pure absorption
of light. In this case ? pure black
This gives Black Body Spectrum pure thermal
spectrum unique spectral shape depends only
on temperature.
Stars approximate thermal spectra
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(13) Two Characteristics
(a) Hotter objects ? curve moves to blue
Weins Law ?peak(nm) 3x106 / T(K)
(b) Hotter objects ? more photons made
Stefan-Boltzmann Law E s T4 E in Watt per
square meter T in Kelvin s is Stefans constant
5.7 x 10-8
Human 300K 10 µm thermal emission
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(14) Atoms Particles
We need to understand these for two reasons
(i) electrons jumping ? colors/light ?
diagnostics (ii) nuclei combing ? powers
stars ? makes elements
(c) protons defines element 1H, 2He,
3Li, . 6C, 7N,. 26Fe, . 92U
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(No Transcript)
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Cosmic abundances for all isotopes of all
elements (relative to Si 106)
a elements
Iron peak
rare earths
light elements
Au
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(e) Electrons 1/2000 mass of p,n ve charge
if e p ? neutral atoms if e ? p
? charged ions (usually fewer e)
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Three types of spectra
consider also graphs of spectra not just bars
Continuous
Emission Line
Absorption Line
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(15) Light ?? Atom interactions
Photons interact quite strongly with electrons in
atoms

• electron jumps down ? photon emitted
• electron jumps up ? photon absorbed

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Energy of photon energy difference
between orbits Ephoton hf
hc/? E4 E3
3
2
1
Single colors emitted/absorbed Color set depends
on orbit set Different for each atom.
c) electron ejected (ionization) ? energetic
photon absorbed
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Emission line spectra from specific atoms
The specific set of lines is unique to each
atom. It is like a bar-code identifier, or a
fingerprint.
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(16) Hydrogen Atom its Spectrum
Atomic orbits are often redrawn as energy level
diagrams
Bohr orbit figure
Energy level diagram
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4
3
2
1
0.0
Hydrogen spectra are particularly simple Several
line series Lyman (UV) Balmer (optical)
Paschen (IR)
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Hydrogen Spectrum
UV Lyman
Visible Balmer
Infrared Paschen
wavelength
1st three lines of the Balmer series Ha , Hß , H?
,
Nebulae often appear pink due to strong Balmer Ha
emission at 656nm from the hydrogen gas.
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(17) Kirchoffs Laws
Define conditions for the formation of spectra
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Star spectra have absorption lines
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(18) Absorption line strengths
The strength of an absorption line depends mainly
on temperature
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(19) Spectral sequence
Oh Be A Fine Girl/Guy Kiss Me
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Detailed Sunlight spectrum with many absorption
lines
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(20) Abundance analysis
Absorption line strength depends on temperature
and abundance how much element is
present Possible to measure/calculate relative
abundances all stars have 74 Hydrogen
24 Helium
2 all others (Star surfaces, not
interiors).
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(21) Doppler Shifts velocity
Moving light source changes apparent
wavelength Towards/away ? blue/red shift Only
radial velocity component, Vr , affects wavelength
Vr/c (? ?0)/ ?0
Binary star in orbit
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(22) Line widths gas pressure
Some stars have wider lines than others. Higher
gas pressure blurs the atom energy levels ?
gives broader line. Dwarf stars ? compressed
atmosphere ? broad lines Giant stars ? fluffy low
pressure atmosphere ? narrow lines
E
E
High pressure Broad line
Low pressure Narrow line