Light%20acts%20like%20a%20Wave

1
Light acts like a Wave
Light can be though of as a propagating
electromagnetic wave. The wave travels at the
maximum allowed speed (c3x108 m/s) through a
vacuum. The electric and magnetic fields are felt
to oscillate at right angles to each other as the
wave passes.
2
Properties of a Wave
The distance between crests or troughs is the
wavelength l. If the wave
travels at speed c, the crests will pass with a
frequency f. The relation between these is l x f
c. People wondered what medium the light
waves are travelling in. Today we just say it is
the vacuum (which isnt totally empty in modern
physics. We also now know that the speed in this
medium is the same no matter how fast the emitter
is travelling (very odd a result of Einsteins
relativity).
3
Light also acts like a particle the Photon
When detectors (or atoms) see light, it arrives
in discreet packages, which we call photons.
You can think of each photon as having a
wavelength. The energy of a photon depends on its
wavelength or frequency E f c / l .
As a whole, the photons have the statistical
behavior of waves of that wavelength. They
experience interference like waves.
4
Light is Electromagnetic Radiation
All wavelengths (or energies) of light are the
same basic stuff. Together they constitute the
electromagnetic spectrum. Visible light is a
tiny portion of this. Although our eyes cannot
detect the rest of the spectrum, we now have
detectors that can. We give different names to
the different colors. We use different units
for the different wavelengths whatever is
convenient.
5
The Spectrum of E-M radiation
Astronomical objects are capable of producing
different parts of the spectrum depending on how
energetic the processes that are going on, or how
hot the object is.
6
The Atmospheric Windows
7
Thermal or Blackbody Radiation
Any opaque body produces E-M radiation
characteristic of its temperature. It follows the
Planck curve shape, which has a peak. The
wavelength of the peak follows Weins law
lmax(nm)3x106 / T(K) (so hotter sources are
bluer). The total energy emitted (or total area
under the curve) by hotter sources of the same
size goes up like T4 , and they are brighter at
all wavelengths.
This is called blackbody radiation because
its what you get from a little hole in a dark
cavity, or a black-looking absorber (which is
also an excellent emitter). A better name is
thermal radiation, because it is related to T.
8
Astro Quiz
Which statement below is FALSE?

• Two stars are the same distance and size, but one
  looks brighter. It must also be hotter.
• Two stars are the same distance and temperature,
  but one looks brighter. It must also be larger.
• Two stars are the same size and temperature, but
  one looks brighter. It must also be bluer.

Reminder The wavelength of the peak follows
Weins law lmax(nm)3x106 / T(K) (so
hotter sources are bluer). The total energy
emitted (or total area under the curve) by hotter
sources of the same size goes up like T4 , and
they are brighter at all wavelengths.
9
Thermal Radiation from Objects
There are also a variety of non-thermal
processes (often involving magnetic fields) which
produce radiation at all wavelengths (and can
produce VERY high energy radiation) all the way
up through gamma rays. They are often associated
with violent phenomena (explosions, black holes,
etc.).
10
Energy Levels in Atoms
We can think of an atom as consisting of a
positive nucleus (protons and neutrons)
surrounded by negative electrons. The electrons
can be thought of as orbiting the nucleus, but
are only allowed in certain orbits (or energy
levels). A photon with exactly the right energy
can excite the electron from one level to
another. The electron will drop back to the
ground state, and emit photons with specific
energies as it does so. Chemistry is caused by
the fact that no 2 identical electrons can be in
the same orbital at the same time.
11
Energy transitions and photons
The energy of photon that can interact with a
level jump just depends on the energy difference
between the levels. Levels can be skipped.
12
Unique Atomic Signatures
Each atom has a specific set of energy levels,
and thus a unique set of photon wavelengths with
which it can interact.
13
The Doppler Shift how it works
When a source is moving, an observer gets the
waves either stretched out or crunched together,
depending on their relative motion with the
source. In the case of light, longer wavelengths
look redder and shorter wavelengths look bluer.
This is given by the Doppler formula
v is negative for an approaching source if the
distance is shrinking, the wavelength is too
To get an appreciable change, you have to be
moving with an appreciable fraction of the speed
of the wave
14
The Doppler Shift how we use it
Atomic energy transitions leave features in the
spectrum whose rest wavelengths are known from
laboratory work. We can measure observed shifts
in these wavelengths from astronomical objects,
and see how fast they are moving (you only get
the line-of-sight motion towards or away from
you).
More subtle analysis can also yield other
motions, like rotation or turbulent motions.
These are all direct uses of the Doppler shift.
It doesnt matter how far away the source is,
either.