- Different phases: Bulk matter can be
in the solid, liquid, or gas form. Sometimes the same substance
can be in any of those three phases, depending on its temperature
(and pressure); Evaporation, melting are examples of phase transitions.
- Why? To
understand, we need to make a model for what matter
is made of,
when we give or take away energy.
- Atoms and molecules: The smallest units
of a simple substance (an element) are atoms, those of a compound substance
are molecules, or combinations of atoms held together by chemical bonds.
- Is there a highest temperature? Probably not; A fourth phase is that of a plasma, ...
- Heat and energy: Heat is energy and produces
random motion. When the temperature rises, atoms or molecules move
faster, flow more easily past each other and take up more space.
Heat can travel,
by conduction, convection or radiation.
|| Structure of Matter
- Structure of an atom: One
model is that of a miniature "solar
system", with protons (+ charge ) and
neutrons in the nucleus, and electrons (– charge) orbiting
- Variation: The number of p's
determines the element (see the periodic table), the number of n's the isotope of that element,
the number of e's whether the atom is a neutral one or
an ion. A gas can become ionized when T is in the 1000s
- The quantum model: All particles are
also waves, which tell us where there is a higher probability of finding
the particle. In an atom or a molecule,
those waves can only fit in certain patterns,
or energy levels. Particles can only be in one of those states, and
jump between them.
- Changing a nucleus: Some nuclei break
up easily, but in general temperatures of millions
of K are needed to produce
reactions that combine or break nuclei.
- Other particles: Other types are produced
in many reactions, like photons, neutrinos... Each type of particle
has an anti-particle.
- How many forces? So far, it boils down to four: Gravitational, electromagnetic,
and the strong and weak nuclear forces.
- What kinds of light
are there? We can see different colors,
which we can separate by separating them into a spectrum, either
with a prism or with a diffraction grating. There are more kinds
of "light", that our eyes can't see, like infrared (IR) and ultraviolet (UV).
- How does it affect
matter? It can be emitted by matter, be
absorbed, reflected, scattered or transmitted by it. It can heat
matter, because light (radiation) carries heat-energy.
- How fast does light
move? Almost instantaneously, but actually
at almost 300,000 km/sec! Usually in a straight line, but there
are exceptions: Reflection, scattering as in atmosphere, bending
in a transparent material, diffraction and
interference – which
tell us that light is a wave!
- What does it tell
us about the source? How far it is, how
it is moving, and also its temperature and what it is made of. To understand how this works, we need to know more
about what light is and where it comes from.
- Relationship: For any wave (sound, water
waves, strings, ...), v = λ f.
| Nature of
- Light is a wave: We know because it shows
interference and diffraction.
Ther waves are oscillating electric and magnetic fields, and can be
produced by oscillating charges.
- Important concepts: Period, wavelength λ,
amplitude, frequency f, speed or velocity v.
- Light is made of particles: We know
because of the photoelectric effect; The particles of light are called
and for any of them, energy = h (frequency), where h is
- So: Light is not that different from matter,
they are all both particles and waves!
| Spectroscopy and Interaction with
- The whole electromagnetic spectrum: If
we consider all wavelengths, the possibilities are radio waves (the longest
ones), microwaves (mm's or cm's), infrared
radiation (microns to mm's), visible light (between
400 and 700 nm), ultraviolet light (down to nm's), X-rays (down
to 10–11 m), and gamma rays (the
- What makes objects emit different types? The
main factor is temperature; For a dense
object that emits a continuous
spectrum, it is the only factor; The hotter it is, the brighter
the radiation and more energetic the photons.
- For thin gases: You get an emission
line spectrum, with wavelengths that depend
on the atoms or molecules, which can emit only certain fixed
amounts of energy.
- Do all waves reach us from space? No,
visible light does, many radio waves do; some IR but not much.
from Light and Radiation
- Different type of spectrum: If
a dense source of light is behind
a thin cooler gas, we get an absorption
spectrum, like the one from the
- Effect of distance: The brightness
of the light decreases in a specific way with distance.
- Effect of motion: There is a Doppler
shift towards the red if the distance is increasing, towards
the blue if it is decreasing.
- Information on the source: We can
often find out what a source of light is made of, how hot it is,
how fast it is moving and spinning,
and/or how far it is from us.Once reflection
from other places has been taken into account!
page by luca bombelli <bombelli at olemiss.edu>,
modified 11 sep 20012