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Light and Atoms

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Title: Light and Atoms


1
Light and Atoms
2
Properties of Light
  • We can come to understand the composition of
    distant bodies by analyzing the light they emit
  • This analysis can tell us about the composition
    as well as the temperature
  • There are limits to what we can learn from the
    ground here on Earth. Why?

3
Properties of Light
  • Radiant Energy travels through empty space
    without a need for a direct physical link
  • Travels at 299,792.5 km/s the constant c (the
    upper limit to all motion)
  • Can circle the Earth in 1/7 of a second
  • The speed of light is reduced when it passes
    through transparent materials like glass, water
    and gases
  • Different colors of light are slowed down
    differently (blue travels more slowly than red)

4
The Nature of Light
  • We have a dual model
  • An electromagnetic wave
  • A stream of particles called photons
  • Electricity and magnetism fluctuate back and
    forth allowing the wave to propagate itself an
    electric field creates a magnetic disturbance
    which in turn creates a new magnetic field
  • The wave model does not work to explain the
    different behavior of light

5
Electromagnetic Waves
6
The Nature of Light
  • Particles of light are called photons
  • Some properties of light are better explained by
    the particle (photon) model
  • We will use mostly the wave model

7
Particle Model
8
Light and Color
  • The visible part of the electromagnetic spectrum
    is what we can see with our eyes
  • The color of the light is determined by the
    wavelength ? (lambda) the distance between wave
    crests
  • Deep red 7 X 10-7 m or 700 nanometers
  • Violet 4 X 10 7 m or 400 nanometers

9
Light and Color
  • The shorter wavelengths tend towards the blue
    (Carry the most energy)
  • The longer wavelengths tend towards the red
  • We usually express these light wavelengths in
    nanometers (nm)

10
Electromagnetic Spectrum
11
Frequency
  • Frequency the number of wave crests passing a
    given point in 1 second
  • Measured in hertz ? (nu)
  • ? ? c

12
White Light
  • Some light seems to have no color
  • White light is a mixture of all colors a blend
    of all the wavelengths of visible light
  • Newton passed white light through a prism which
    split the white light into all the colors of the
    spectrum He also recombined all the colors by
    passing them through a lens and reconstituted the
    white light

13
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14
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15
Infrared
  • Infrared just beyond the red
  • Infrared discovered by Sir William Herschel
  • Infrared wavelength is longer than visible light

16
Ultraviolet
  • Shorter wavelength than the visible
  • Discovered in 1801 by J. Ritten

17
Radio Waves
  • Predicted by James Clerk Maxwell in the
    mid-1800s.
  • Produced experimentally by Heinrich Hertz in 1888
  • Discovered coming from the cosmos by Karl Jansky
    in 1930s

18
Radio Waves
  • Range in length from a few millimeters to
    hundreds of meters
  • Communications
  • Radar
  • Microwave ovens
  • Radio telescopes
  • SETI

19
X-rays
  • Discovered by William Roentgen in 1895
  • Detected in space in 1940
  • Shorter wavelengths than visible light
  • Help detect black holes

20
Gamma Rays and Region between Infrared and Radio
Waves
  • Regions not well explored
  • Both of these areas are blocked by Earths
    atmosphere making it difficult to study them form
    Earth

21
Energy carried by EM waves
  • Different wavelengths carry different amounts of
    energy
  • E hc speed of light c and h are constant
  • ?
  • An inverse proportion if wavelength increases
    energy decreases if wavelength decreases energy
    increases (why UV light gives you sunburn and IR
    does not)

22
Wiens Law
  • The wavelength at which a body radiates most
    strongly is inversely proportional to the bodys
    temperature (hotter bodies radiate more strongly
    at shorter wavelengths)
  • Using this law we can now measure how hot an
    object is simply from the color of the light it
    radiates most strongly
  • This law is fairly accurate for most stars and
    planets

23
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24
Using Wiens Law
  • If we know the wavelength of the strongest
    radiation from a body we can determine its
    temperature
  • T (K) 3 X 106/ ?
  • What is the surface temperature of a star that
    emits light the strongest at 300 nm?
  • 3000000300 10,000 K

25
Black Bodies
  • Black Body an object that absorbs all of the
    radiation falling upon it
  • It reflects no light
  • They radiate more efficiently than any other type
    of body
  • Very few objects are perfect black bodies
  • Most objects we study in space are close enough
    to black bodies to obey Wiens law with little
    error

26
The Structure of Atoms
27
Atomic Structure
28
Formation of a spectrum
  • Spectroscopy breaking light down into its
    component parts
  • Each atom has a spectral signature of certain
    amounts of light present at each wavelength
  • Electrons moving from one orbital to another
    produce different kinds of light
  • When electrons fall from a higher energy level to
    a lower energy level they give off light
  • Sodium Yellow light, Strontium red light,
    Copper green light are some examples

29
Emission spectrum
30
The Doppler Shift
31
Absorption in the Atmosphere
  • Gases in the atmosphere affect the flow of heat
    and light
  • Very little visible light is absorbed
  • Infrared and UV are strongly absorbed by carbon
    dioxide and water
  • X-rays and gamma rays are strongly absorbed by
    oxygen and nitrogen
  • No EM radiation of wavelengths shorter than 300
    nm reach the Earth

32
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33
Light in the Atmosphere
  • Refraction and Dispersion
  • Refraction distorts the Suns shape when it rises
    and sets
  • Also makes the stars twinkle
  • Dispersion make the flashing colors seen in
    twinkling stars

34
The Moon Illusion
  • Why does the moon appear larger sometimes
  • Not fully understood, but it is an optical
    illusion based on what our mind perceives the
    relationship is to the objects in the foreground

35
Twinkling Stars
  • Called scintillation
  • Caused by differences in the air density through
    which the starlight is passing due to subtle
    temperature differences in the atmosphere

36
Atmospheric Scattering
  • Creates the blue color of the sky during daylight
  • If there were not atmosphere to scatter the light
    the daytime sky would be black
  • Atmosphere hardly affects the red wavelengths of
    light
  • Blue scatters strongly
  • Large particles scatter light evenly clouds are
    white
  • Small molecules (like nitrogen and oxygen)
    scatter shorter wavelengths like blue
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