Chapter 5
Properties
of Light
 Range of wavelengths. - The EM Spectrum
- Particle or Wave
-- the "duality" question
Properties
of Matter
- By E=mc2,
matter is "frozen" energy ("mass" is just the quantity of matter in a
body)
- "mass and energy are both but
different manifestations of the same thing " Albert Einstein
- A little mass can be converted
into an enormous amount of energy
- Matter is composed of
atoms
- Atoms are composed particles
of mass with various particles
- The three most common and
important particles are:
- Protons, which are found in the
nuclei of atoms and are positively charged
- Neutrons, which also are found in
the nuclei of atoms and have no charge
- Electrons, which are found at
various specific distances from the atomic nucleus and which have a
negative charge
- Elements differ from each other in the number of
protons in their nuclei
- For example, Hydrogen has one proton
(and one electron)
- More massive Helium has two protons,
two neutrons and two electrons
- Atoms of the same element with
differing numbers of neutrons are called "isotopes"
- Atoms of the same element with
differing numbers of electrons are called ions (electrons are negative
ions)
- Atoms display levels
of energy characteristic of their specific element, temperature and
state of compression
- Uncompressed (as in a gas)
atoms of different elements can have only very specific energy levels
- Compressed atoms (tighly
packed as in a solid or liquid) can exhibit an virtual infinitude of
energy levels
- Energy levels here correspond
to the energy of electrons moving around the nucleus.
- These levels are related to
spectral formation (see Spectral Formation below)
Light
and Matter
- Thermal Radiation
- The radiation of any body that
depends on the body's temperature
- The human body, for instance,
usually gives off infrared radiation
- This is also called "blackbody
radiation", a rather unfortunate term that does NOT designate color
- Spectral Formation
- Atoms are small! -- An atom is about
10-11 meters, whereas just the nucleus is roughly 10-15 meter. For
instance, a hydrogen atom is about as big compared to a racquetball, as
the racquetball is to the Earth. The human body has about 3.4 times
1027 atoms (that's 3.4 billion billion billion atoms!).
 The way atoms vibrate determine what
color of light (otherwise known as wavelength) they produce. The color
of light or wavelngth they produce is dependent on temperature. Thus
color is associated with temperature. [e.g., red stars are less hot,
yellow stars are moderately hot, and blue stars are the
hottest.] This is called Wien's Law.
- In an uncompressed gas, only very
specific energy levels are permissable, and these vary with the
element. Electrons move between these very specific levels in what are
called "Quantum Leaps." When they do this, they give off or absorb
specific photons of light (you can think of them as particles of light
with specific wavelengths.
- In this crude diagram, electrons could exist in
levels 1, 2, 3 or higher, but not in between -- levels such as 1/2. 3/4
and so on. Only certain levels are "permitted."
- Kirchhoff's Laws
- A heated solid, liquid or compressed gas
gives off a continuous spectrum when heated or energized.
- Each non-compressed gaseous element, say
hydrogen, when heated or otherwise energized, will give off a
distinctive series of colors that can be separated in a spectroscope.
(This produces an emission spectrum. We indicated non-compressed
because if it is compressed or at high pressure, it will give off a
continuous spectrum.) If we observe light with an emission pattern, we
know that it was a gas that produced it.
- In the reverse manner, if you pass light
through a non-compressed gas such as hydrogen (one that is cool and not
producing its own light), that gas will remove light from the beam at
the same wavelengths that it would produce if heated. Thus a beam of
light that is missing a particular pattern of wavelengths gives
evidence that the light was passed through a particular gas. (This
produces an absorption spectrum.)
We observe particular chemical
elements in stars through these patterns in their light . It
is
exactly this process whereby we know the chemical compositions of the
Sun and stars. Light from a hot, highly condensed gas (the surface of
the Sun or a star) passes through a cooler,less compressed gas (the
atmosphere of the Sun or a star) and specific wavelengths of light
corresponding to specific elements are filtered out. We see these as
the dark lines in an absorption spectrum. The exact positioning of the
lines gives their wavelengths, which reveals their nature. Here is the
spectrum of the Sun:
- Doppler Shift
- Change in wavelength
due to motion of the source or the observer or both
- When the distance
between source and observer is increasing, the waves become longer
(redder)
- When the distance
between source and observer is decreasing, the waves become shorter
(bluer)
- Works with all forms
of radiated wavelength energy, including sound
- In astronomy, this
is simply called "Red Shift" when it has to do with the expansion of
the Universe
Telescopes
- Types
 Refractors
- Credit for invention goes to Dutch
spectacle maker Hans Lippershey in 1608
- Uses a large lens to collect light
- Limited in size due to nature of the
glass lens
- Large ones are far more expensive than
reflectors that use mirrors
- Uses a large mirror to collect light
- Can be made very large much less
expensively than refractors
- The world's largest optical telescopes
today are of this type
-
 Radio
- Collects long wavelength radio waves
and typically needs much larger collector than optical telescopes
- All radio telescopes use some kind of
antenna. The most common type is a large metallic mesh dish
- Other Types
None of
these are telescope in the traditional sense, and they certainly don't
look like telescopes. They are detectors looking for waves and
particles from space, not so much to form an image as to count or
simply confirm existence of these waves and particles.
- Gravitational Waves. These are theorized to
propagate when massive objects such as black holes collide in deep
space. They are thougth to be vibrations in the very "fabric of space"
itself. One way is to watch (very carefully) for unexpected vibrations
in a mass, which might be caused by a passing gravity wave (which, by
the way, is extremely weak). One such experiment called LISA
involves satellites measuring in space so as to avoid the many
vibrations and interference on the Earth.
- Neutrino. There will be lots more about this
when we talk about the Sun. Basically, a neutrino is a tiny, low mass
particle produced in the nuclear reactions in stars. It is not
radioactive or dangerous in any way because it is so incredibly
difficult to detect -- it just does not interact well at all. But by
studying it, we learn more about the internal conditions and processes
in the Sun and other stars.
- Other. Astronomers study other wavelengths
(ultraviolet, X-ray and gamma rays) as well as other particles from
space (such as cosmic rays, which are not really "rays" at all but
hihg-energy particles).
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