Light and Atoms
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?
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)
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
Electromagnetic Waves
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
Particle Model
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 = λ (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
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)
Electromagnetic Spectrum
Frequency
• Frequency: the number of wave crests
passing a given point in 1 second
• Measured in hertz = ν (nu)
• λν=c
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
Infrared
• Infrared – just beyond
the red
• Infrared discovered by
Sir William Herschel
• Infrared wavelength is
longer than visible
light
Ultraviolet
• Shorter wavelength
than the visible
• Discovered in 1801 by
J. Ritten
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
Radio Waves
• Range in length from a few millimeters to
hundreds of meters
• Communications
• Radar
• Microwave ovens
• Radio telescopes
• SETI
X-rays
• Discovered by
William Roentgen in
1895
• Detected in space in
1940
• Shorter wavelengths
than visible light
• Help detect black
holes
Gamma Rays and Region between
Infrared and Radio Waves
• Regions not well
explored
• Both of these areas are
blocked by Earth’s
atmosphere making it
difficult to study them
form Earth
Energy carried by EM waves
• Different wavelengths carry different
amounts of energy
• E = hc speed of light c and h are constant
λ
• An inverse proportion – if wavelength
increases energy decreases – if wavelength
decreases energy increases (why UV light
gives you sunburn and IR does not)
Wien’s Law
• The wavelength at which a body radiates most
strongly is inversely proportional to the body’s
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
Using Wien’s Law
• If we know the wavelength of the strongest
radiation from a body we can determine its
temperature
• T (K) = 3 X 106/ λ
• What is the surface temperature of a star that emits
light the strongest at 300 nm?
• 3000000÷300 = 10,000 K
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 Wien’s law with little error
The Structure of Atoms
Atomic Structure
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
Emission spectrum
The Doppler Shift
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
Light in the Atmosphere
• Refraction and Dispersion
• Refraction distorts the Sun’s shape when it
rises and sets
– Also makes the stars twinkle
• Dispersion make the flashing colors seen in
twinkling stars
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
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
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