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Creating Light
Light as a Wave
Light (or electromagnetic radiation), can be thought of as either a
particle or a wave. As a wave, light has
• a wavelength,  (distance between waves)
• a frequency,  (number of waves passing you each second)
• a speed, c =   (this is always the same: 300,000 km/s)
• an energy, E = h  (where h is just a constant)
Note that because the speed of light is a constant, , , and E are
linked: if you know one, you know the other two.
Light as a Particle
Light can also behave as particle. Each packet of light is called a
photon, and each photon carries a specific amount of energy
(associated with the photon’s wavelength or frequency).
Photons emitted from a source
will spread out in all directions
at the speed of light. Since the
amount of area surrounding a
source increases as the distance
squared, the density of photons
will decrease as 1 / r2. This is
the inverse square law of light.
Ways of Creating Light
There are 3 ways to produce light.
• Through the blackbody process (a.k.a. thermal
emission)
• Through line emission
• Through synchrotron emission
(This last way is only for a few peculiar objects with strong magnetic
fields. We will be ignoring this mechanism in this class.)
The Blackbody Process
Anything that is hot (i.e., above absolute zero) produces light at all
wavelengths – a continuous spectrum. But the amount of light given
off at each wavelength is very sensitive to the object’s temperature.
Specifically,
The hotter the object:
• the more high-energy
photons are created
• the more light is
created (MUCH more)
L  T4
The Blackbody Temperatures
Temperatures
Sun: 6000° (optical)
People: 300° (IR)
Rigel: 44,000° (UV)
1,000,000° gas: x-ray
(all temperatures are
in Kelvin)
The Bohr Model of the Atom
In the Bohr model of the atom, the nucleus contains protons and
neutrons. Circling around the nucleus (in orbitals) are electrons.
Since electrons are attracted to the protons, they normally orbit in
their lowest energy state (i.e., closest to the nucleus).
Important: electrons can
only orbit at very specific
distances from the nucleus.
The exact positions of the
orbitals are different for
every element.
Creating an Emission Line
Suppose something collided into an electron orbiting in the
lowest energy level. Some of the energy of the collision could
kick the electron up to a higher level. Eventually, when the
electron falls back down, it has to give this energy back. It
does so by giving off light.
Since each orbital has a
very specific level,
electron transitions
between the orbitals
emit very specific
amounts of energy. The
spectrum from this
process would not be
continuous.
Emission and Absorption
An electron does not have to be collided up to the higher orbital. If a
photon comes by with an energy that is exactly equal to the difference
between the two levels, the electron can absorb the photon, and use the
energy to jump to the higher level.
The electron would, of course,
later fall back down and re-emit
the photon. (But not necessarily
in the same direction.)
Ionization and Emission
Suppose a very high energy photon passes near an atom. If the
photon has enough energy, it can kick the electron completely out of
the atom, and create an ion. Eventually, the electron will recombine
into (some level of) the ion, and cascade its way down to the lowest
energy level. Each downward transition will produce a photon with
the exact energy of the transition. This is not a continuous spectrum!
There is light only at very specific energies (i.e., colors), which
correspond to each transition.
Emission Line Spectra
Since every element has a different set of atomic orbital energies,
each element creates a different emission line spectrum. They are as
unique as fingerprints!
Absorption Line Spectra
An object (like a star) emits a hot blackbody spectrum. Somewhere
between you and the star (like on the outside of the star) is some
cooler gas. That gas can absorb the photons which correspond to the
atom’s energy levels. The result is an absorption spectrum.
You observe the
blackbody spectrum
minus the energy that has
been absorbed by the gas.
(These photons have
been re-emitted, but in
other directions).
Emission versus Absorption
Atomic emission and absorption are really two sides of the same coin.
Photons that are absorbed can be re-emitted to produce an emission
line spectrum.
Emission versus Absorption
Atomic emission and absorption are really two sides of the same coin.
Photons that are absorbed can be re-emitted to produce an emission
line spectrum.
Telescopes
Types of Telescopes
The purpose of a telescope is to collect as much light as
possible and focus it into a small area. This can be done
in 2 ways
• By using a lens to bend, or refract the light
• By using a mirror to reflect the light
Telescopes are characterized by their collecting area:
a 36-in telescope has a lens (or mirror) that is 36-inches
in diameter.
Atmospheric Seeing
The focal length of a telescope is the distance from its lens (or
mirror) to the focus. In general, the larger the focal length, the
larger the magnification. However, the atmosphere of the Earth
blurs out all images. (This “barbeque on a hot day” effect is
called “seeing”.)
Since the best
seeing on Earth is
slightly less than 1
arcsec, there is no
point in trying to
magnify any more
than this.
Refracting Telescopes
Refracting telescopes use a lens to bring the light to a
focus. Light passes through the primary to the eyepiece.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Refracting Telescopes
Refracting telescopes are usually small, because
• It is difficult to physically support a big lens
• Light going through glass only gets bent a little, so large
refractors have very long focal lengths. The result is very
high magnifications, and physically large structures.
• When light passes
through glass, the blue
light gets bent more
than red light. So the
red-light focus of a
large refractor is at a
different place from the
blue-light focus. This is
chromatic aberration.
Reflecting Telescopes
Reflecting telescopes use mirrors to collect the light. They can be
REALLY BIG, but the focus is high in the air. To solve this
problem, one can place smaller mirrors (secondaries) in the beam.
To use a reflecting telescope,
one could:
• Observe at prime focus
• Place a small pick-off mirror
near the top, and observe off
to the side (Newtonian)
• Reflect the light back down
through a hole in the
primary (Cassegrain)
• Make multiple reflections
and observe somewhere
comfortable (Coudé)
Reflecting Telescopes
Mirrors are not 100% reflective, so every reflection loses some
light. Ideally, one would like to observe at prime focus, but for
practical reasons, Cassegrain and Newtonian focii are often used.
Where to Put a Telescope
For an optical observatory, you need
• Dark Skies
• Stable air (for good seeing)
• Good weather
• High altitude (to get above as much of the atmosphere
as possible). This is especially important if you also
want to use the telescopes to observe in the infrared.
For a radio observatory, you need
• An area far from cell phones and TV/radio stations
Observatories
Mauna Kea
ESO
McDonald
Kitt Peak
VLA
Atmospheric
Windows
For gamma-rays, x-rays and the ultraviolet, one has to observe
from space. For the radio and optical, the ground is fine (except
that the atmosphere blurs out optical light).
The infrared is tricky -- there are some “atmospheric windows”,
where observations are possible. (Most things on Earth also glow
in the IR.) But in general, the higher up you are, the better.
Space Observatories
Chandra
Hubble
Galex
Spitzer
WMAP
Refracting Telescopes
Reflecting Telescopes
Prime Focus Observations
Newtonian Focus Telescopes
Cassegrain Focus Telescopes
Coudé Focus Telescopes
Next time -- REVIEW