This Set of Slides
• This set of slides deals with the nature of
light, how it’s created, some ways that it’s
used in astronomy.
• Units covered: 21, 22, 23, 24 and 25.
Light
• In Astronomy, it is far too difficult to visit
stars and most planets in person.
• Astronomers’ primary tool in learning about
the universe is from the electromagnetic
radiation, or light, that we can detect.
• To understand how astronomers know what
they know, you must understand some things
about electromagnetic radiation - light.
The Nature of Light
• Light is radiant energy.
• Travels very fast –
300,000 km/sec,
186,000 miles/sec
• Has a dual nature - Can be
described either as a wave
or as a particle traveling
through space.
•
As a wave…
– A disturbance in an electric field creates a
magnetic field, which in turn creates an electric
field, and so on, a self-propagating
electromagnetic wave.
– Light waves can constructively or destructively
interfere.
– The color of light is determined by its
frequency.
– The energy is also determined by frequency.
•
As a particle…
– Particles of light (photons) travel through
space.
– These photons have very specific energies,
that is, light is quantized.
– Photons strike your eye (or other sensors)
like very small, massless “balls” (maybe
bb’s), and are detected.
Light as a Wave Versus Mechanical Waves
• Wave – transfer of energy without the transfer of matter.
• Wave phenomena – refraction, diffraction, constructive and
destructive interference, superpositioning, Doppler shift.
• Measurable wave characteristics – amplitude, wavelength,
frequency, period.
• Mechanical Waves – water, sound – must have some physical
matter – a medium – in which to exist and travel.
• Light exhibits all wave phenomena and has all the measurable
wave characteristics (as a mechanical wave).
• BUT, light does not require any physical matter for its transfer.
Light can exist and travel through the vacuum of space.
Wavelength
• The colors we see are determined by
the wavelength of light.
• Wavelength is the distance between
successive crests (or troughs) in an
electromagnetic wave.
• This is very similar to the distance
between the crests in ocean waves.
• We denote the wavelength of light by
the symbol .
• Wavelengths of visible light are very
small.
– Red light has a wavelength of 710-7
meters, or 700 nanometers (nm).
– Violet light has a wavelength of
410-7 meters, or 400 nm.
– Colors in between red and violet
(remember ROY G BIV?) have
intermediate wavelengths.
Frequency
• Sometimes it is more
convenient to talk about light
in terms of frequency, or how
fast successive crests pass by
a given point.
• You can think of frequency
as a measure of how fast you
bob up and down as the
waves pass by.
• Frequency has units of Hz
(hertz), and is denoted by the
symbol . 1 Hz = 1 cycle/sec.
• Long wavelength light has a
low frequency, and short
wavelength light has a high
frequency.
• Frequency and wavelength are related by:
   c
Where ‘c’ is the speed of light.
White Light
• Light from the Sun
arrives with nearly all
wavelengths, and we
perceive this mixture of
colors as white.
• Newton demonstrated
that white light could be
split into its component
colors with a prism, and
then recombined into
white light with a lens.
The Electromagnetic Spectrum I
• There is much more to light
than just visible light, the light
that humans can see.
– Radio waves have very
long wavelengths, as much
as a meter and more.
– Microwaves (like the ones
we cook with) are at the
upper end of the radio part
of the spectrum.
– Infrared wavelengths are
longer in wavelength than
visible light.
The Electromagnetic Spectrum II
• Above the visible…
– Ultraviolet waves are
shorter in wavelength
than visible waves.
These included the waves
that tan or burn us.
– X-rays come mostly from
stellar sources in nature,
and can penetrate many
materials, like skin,
muscle and bone.
– Gamma rays have the
shortest wavelengths.
Energy Carried by Photons
• A photon carries energy with it
that is related to its wavelength
or frequency:
E
hc

 h 
• From this we see that long
wavelength (low frequency)
photons carry less energy than
short wavelength (high
frequency) ones. This is why
UV waves give us a sunburn,
and X-rays let us look through
skin and muscles.
The Nature of Matter
• An atom has a nucleus at
its center containing
protons and neutrons.
• Outside of the nucleus,
electrons move in
“clouds” called orbitals.
– Electrons can also be
described using wave or
particle models.
– Electron orbitals are
quantized – that is, they
exist only at very specific
energies.
– The lowest energy orbital
is called the ground state.
• To move an electron from one orbital to the
next higher one, a specific amount of energy
must be added. Likewise, a specific amount
of energy must be released for an electron to
move to a lower orbital.
• These are called electronic transitions.
Some Quantum versus Classical Mechanics
• An early (circa 1900) atomic model was equivalent to a
planetary model – the nucleus was considered to be like
the Sun with the planet-like electrons in orbits.
• This model didn’t last long.
• An object (planet, moon, artificial satellite, space station)
can be in orbit at any level as long as the speed is right.
• An electron in an atom can not be in any “orbit” but only
in very well-defined orbital levels.
• An electron moves from one orbital to another without
actually passing anywhere in-between! Another “oddity”
of quantum mechanics!
The Chemical Elements
• The number of protons (atomic number) in a nucleus
determines what element a substance is.
• An atom that is neutrally charged has a number of
electrons equal to the number of protons.
• The electron orbitals are different for each element,
and the energy differences between the orbitals are
unique as well.
• This means that if we can detect the energy emitted
or absorbed by an atom during an electronic
transition, we can tell what element the atom
belongs to, even from millions of light years away!
Absorption
• If a photon of exactly the
right energy (equal to the
energy difference
between orbitals) strikes
an electron, that electron
will absorb the photon
and move into the higher
orbital.
– The atom is now in an
excited state.
• If the photon energy
doesn’t match any of the
orbital-energy differences
it can not be absorbed – it
will pass through. We say
the element is transparent
to those frequencies or
colors.
• This process is called absorption.
• If the electron gains enough energy to
leave the atom entirely, we say the
atom is now ionized, or is an ion.
Emission
• If an electron
drops from one
orbital to a lower
one, it must emit
a photon with the
same amount of
energy as the
orbital-energy
difference.
• This is called
emission.
Emission Spectra
•
Imagine that we have hot
hydrogen gas.
– Collisions among the
hydrogen atoms cause
electrons to jump up to
higher orbitals, or energy
levels.
– Electrons can jump back to
lower levels, and emit a
photon of energy h x f.
– If the electron falls from
orbital 3 to orbital 2, the
emitted photon will have a
wavelength of 656 nm.
– If the electron falls from
orbital 4 to orbital 2, the
emitted photon will have a
wavelength of 486 nm.
• We can monitor the light emitted, and
measure the amount of light of each
wavelength we see. If we graph this
data, we’ll see an emission spectrum.
Seeing Spectra
• Seeing the Sun’s spectrum
is not difficult.
– A narrow slit only lets
a little light pass.
– Either a grating or a
prism splits the light
into its component
colors.
– If we look closely at
the spectrum, we can
see dark lines. These
correspond to
wavelengths of light
that were absorbed.
Emission spectrum of hydrogen
• This spectrum is
unique to
hydrogen.
• If we were looking
at a hot cloud of
interstellar gas in
space, and saw
these lines, we
would know the
cloud contained
hydrogen.
Different atom, different spectrum!
• Every element has
its own spectrum.
Note the
differences
between hydrogen
and helium
spectra below.
A spectrum
is like a
chemical
fingerprint!
Absorption Spectra
• What if we had a cloud of cool
hydrogen gas between us and a
star?
– Photons of energies that
correspond to the electronic
transitions in hydrogen will be
absorbed by electrons in the
gas.
– The light from those photons is
effectively removed from the
spectrum.
– The spectrum will have dark
lines where the missing light
would be.
– This is an absorption
spectrum.
– Also unique for each element.
Types of Spectra - Summary
* If the source emits light that is
continuous, and all colors are
present, we say that this is a
continuous spectrum.
* If the molecules in the gas are wellseparated and moving rapidly (have
a high temperature), the atoms will
emit characteristic frequencies of
light. This is an emission-line
spectrum.
* If the molecules of gas are wellseparated, but cool, they will absorb
light of a characteristic frequency as
it passes through. This is an
absorption line spectrum.
Spectra of Astronomical Objects
Measuring Temperature
• It is useful to think of temperature
in a slightly different way than
we are accustomed to.
– Temperature is a measure of the
motion of atoms in an object.
– Objects with low temperatures
have atoms that are not moving
much.
– Objects with high temperatures
have atoms that are moving around
very rapidly.
• The Kelvin temperature scale was
designed to reflect this
– 0  K is absolute zero –the atoms in
an object are not moving at all.
The Blackbody Spectrum
• As an object (piece of iron for
example, or the gas in a star) is
heated, the atoms in it start to
move faster and faster.
– When they collide, they emit
photons with energy
proportional to how hard they hit
• Some collide lightly, and
produce long-wavelength
radiation.
• Some collide very hard, and
produce short-wavelength
radiation.
• Most are somewhere in
Gentle collisions
between.
– As the body gets hotter, the
number of collisions
increase, and the number of
hard collisions increase.
Hard collisions
Results of More Collisions
• Additional collisions mean
that more photons are emitted,
so the object gets brighter.
• Additional hard collisions
means that more photons of
higher energy are emitted, so
the object appears to shift in
color from red, to orange, to
yellow, and so on.
• Of course we have physical
laws to describe these effects.
Wien’s Law and the Stefan-Boltzmann Law
• Wien’s Law:
– Hotter bodies emit more
strongly at shorter
wavelengths. The hotter it is,
the shorter the wavelengths.
• SB Law:
– The luminosity of a hot body
rises rapidly with
temperature.
Taking the Temperature of Astronomical Objects
• Wien’s Law lets us estimate
the temperatures of stars
easily and fairly accurately.
• We just need to measure the
wavelength (max) at which
the star emits the most
photons.
• Then,
T
2.9 106 K  nm
max
The Stefan-Boltzmann Law
• If we know an object’s
temperature (T), we can
calculate how much energy
the object is emitting using
the SB law:
L  T
4
•  is the Stefan-Boltzmann
constant, and is equal to
5.6710-8 Watts/m2/K4
• The Sun puts out 64 million
watts per square meter – lots
of energy!
The Effect of Distance on Light
• Light from a distant source
seems very dim. Why?
– Is it because the photons are
losing energy?
– No – the light is simply
spreading out as it travels from
its source to its destination.
– The farther from the source you
are, the dimmer the light seems.
– The object’s brightness, or the
amount of light received from a
source, decreases with increased
distance. The relationship is
mathematical.
Brightness 
Total Light Output
4d 2
This is an inverse-square law –
the brightness decreases as the
square of the distance (d) from
the source.
Doppler Shift in Sound
• You have experienced the
Doppler shift in sound.
– Standing on the sidewalk,
listening as cars go past.
– As a car approaches, the
sound from the car seems to
have a higher pitch – this is
due to shorter wavelengths.
– As the car passes, the sound
shifts to lower pitch due to the
longer wavelengths.
– Police radar guns work on the
same principle. The waves
reflected off the car will be
shifted by an amount that
corresponds to the car’s speed.
Doppler Shift in Light
• If an object emitting light is
moving toward you, the light you
see will be shifted to shorter
wavelengths – toward the blue
end of the spectrum. We say the
light is blue-shifted.
• Likewise, if the object is moving
away from you, the light will be
red-shifted.
• If we detect a wavelength shift of
 away from the expected
wavelength , the radial (line-ofsight) velocity of the object is:
VR 


c