2 LIGHT

1
2 - LIGHT ASTROPHYSICS
2
Isaac Newton
3
White light is a collection (summation) of colors
that a prism separates
Dispersion - separation of light by color
4
1802 - Thomas Young - light behaves like a wave

• Constructive Interference
• Destructive Interference
• Wavelength ?
• Frequency f or ?
• Speed v

5
Light is a Wave!
Wavelength ? - Units meters (m), ?m,
nm Frequency ? or f - Unit cycles/s, universally
called Hertz (1 Hertz 1 cycle/s) Speed - Units
m/s NOTE v f ?
6
The Speed of Light
Galileo - lanterns Olaus Roemer - Moons of
Jupiter Fizeau Foucault - in the
lab Aberration of Starlight
300,000 km/s (3x105 km/s)
7
Maxwell - Light is a electromagnetic wave with f
? c c speed of light in a vacuum In other
media, vc/n nindex of refraction Vacuum
n1.00000 Air n1.000 Water n1.3 n depends on
?, giving rise to dispersion
8
Other Colors of Light Heinrich Hertz -
produces detects radio waves William Herschel
discovers infrared light
9
The Electromagnetic Spectrum
10
Polarized Light
11
Blackbody Radiation
12
The Ultraviolet Catastrophe
13
Solution - Quantization of Light
1900 - Plank - Light is emitted by atoms with
specific energies E hf or h? 1905 - Einstein -
Light is quantized into specific packets or
particles which we now call photons
Light behaves like a wave and a particle!
14
Cavities Blackbodies
15
Blackbody spectrum depends on Temperature - which
temperature to use? - Fahrenheit, Celsius, or
Kelvin? 0 100 F ice-alcohol mix human
body C water freezes water boils (sea
level!) K motion stops (same scale as C)
16
Laws of Blackbody Radiation
Wien's Law Wilhelm Wien, in 1893
17
Wien's Law - Example
18
Color Temperature
For non-BBs, can measure I?1 and I?2 and ask
What BB has that ratio? Assign the BBs T to the
object as the Tcolor
19
Stefan-Boltzmann Law - 1878-1884 Josef Stefan and
Ludwig Boltzmann
20
Stefan-Boltzmann Law
21
Continuum Lines 1814 - Joseph Fraunhofer
discovers spectral lines in the spectrum of the
Sun.
22
1859 - Robert Bunsen Gustav Kirchhoff every
chemical element, when heated, gives off a unique
set of bright spectral lines
Calcium Iron Sodium
23
Kirchhoffs laws 1. A hot, glowing (condensed)
object emits a CONTINUOUS spectrum. 2. When a
heated gas is viewed against a dark, cool
background (i.e., not a bright continuum), BRIGHT
EMISSION LINES are seen. 3. When a continuous
spectrum is viewed through a cooler gas, DARK
ABSORPTION LINES appear in the continuous
spectrum.
24
Spectroscopy and Atomic Structure
1910 Ernest Rutherford - 99.98 of mass of the
atom is in a small dense nucleus.
25
1885 J.J. Balmer finds that the spectrum of
hydrogen follows a precise mathematical relation
26
1914 Niels Bohr derives structure H atom from
quantized angular momentum
Each element has a different orbital structure
and spectrum
27
Photon-Atom Interactions
28
Molecular Spectra
Rotational Vibrational Electronic
29
SiO
30
Earths Atmosphere
31
(No Transcript)
32
Inverse-square Law of Light
33
Doppler Effect