The textbook web resources for Chapter 3 are
Light...The Doppler Effect
radiant energy - it can travel through space in waves.
electromagnetic radiation - it has characteristics of
electric and magnetic energy.
Light has properties of both a
wave and a particle. This is called having a wave-particle
duality. When it acts like a wave, we refer to is as an
electromagnetic wave. When it acts like a particle, we talk
about photons (particles of energy).
characteristics of light are:
brightness/intensity (the amount of energy in
the light) - this
refers to the height of the wave (wave model) or the
photons (particle model).
- color - this
is determined by the wavelength of the light.
is the distance between the crests (tops) of two adjacent waves. For visible
light, this is usually measured in nanometers (nm - 1 billionth of a meter), but
can also be measured in micrometers (mcm - 1 millionth of a meter) or angstroms
(1 ten-billionth of a meter).
Red Light has a wavelength of about
Violet Light has a wavelength of about 400nm.
usually denote by the Greek letter lambda (λ), which looks like an upside-down
Frequency is the number of wave crests that pass
per unit of time. It is generally measured in Hertz (Hz) and is denoted by the
Greek letter nu (which looks like a lower-case "v" in italics).
speed of light in empty space is a constant: 300,000km/s. This
is denoted by the letter c. The speed of light is the
universal speed limit - nothing travels faster than c.
Because c is
constant, the relationship between wavelength and frequency is c =
wavelength x frequncy.
White light is light
that contains a mixture of all the colors. There is no dominant color, so they
all add together to make white. The spectrum of visible light
that we see contains red, orange, yellow, green, blue, indigo, and violet. The
red light has the longest wavelength of these colors, violet has the shortest.
As a general rule shorter wavelengths are more energetic, longer ones are less
The electromagnetic spectrum contains the
visible light that we see, but also many other types of radiant
energy. These differ by their wavelength. These include (from longest
wavelength to shortest)...
Short-wavelength radiation carries proportionally more
energy than long-wavelength radiation. This means that Gamma Ray end
of the spectrum can carry more energy than the Radio Wave end. So, the energy
of a photon (E) at a given wavelength is equal to Planck's
constant (h) (6.63x10^-34 joule-seconds) times the speed of
light (c), divided by the wavelength
(lambda). This means that hc = 1.99x10^-25
Where this is applicable to everyday life is that
ultrviolet light has enough energy to break atomic and molecular bonds (which is
why it can cause sunburns and damage your DNA - causing skin cancer) but
infrared does not (so it can't cause sunburns or skin cancer).
an object's temperature increases, the object radiates light more strongly at
shorter wavelengths. In other words, if an object is heated up enough
to give off light, it will start off glowing red and progress across the
spectrum towards blue as it gets hotter.
(/Veenz/) Law states that the wavelength at which an object
radiated light most strongly is inversely proportional to its
Temp = 12,000K, Wavelength = 250nm (blue)
6,000K, Wavelength = 500nm (yellow)
Temp = 1,000K, Wavelength =
A blackbody is an object if the radiation
it emits is dependant only on its temperature - not on other factors, such as
its composition. Stars are a good example of blackbodies because the wavelength
(color) of light they emit depends on their temperature. The peak wavelength
emitted is calculated using Wein's Law and this formula:
wavelength = 2.9x10^6 / temperature in Kelvin
Check out the
- it explains this concept well.
Matter is made up of atoms. Atoms have
1. Proton - Has a positive (+) charge, and is
found in the nucleus
of the atom. Has a mass of 1 amu (atomic mass
2. Neutron - Has a neutral (o) or no charge, and
is found in the
nucleus of the atom. Has a mass of 1 amu.
Electron - Has a negative (-) charge. Electrons orbit
nucleus in electron shells (sometimes called electron clouds).
Has a mass of 1/2,000 amu.
An element is a substance
that contains only one type of atom. Elements are organized into the Period
Table, which sorts them by size. You can download a Periodic Table here.
Periodic Table is set up in such a way that it tells you about
the elements and their atoms. Each period (horizontal row) represents an
electron shell. The first period has two elements in it - which tells you that
the first electron shell can only hold two electrons.
Each element in the
Periodic Table has a chemical symbol (a letter or two that
represents that element), an atomic number (which tells you the
number of protons in an element's atom), and an atomic mass
number (which tells you the number of protons and neutrons combined in
Electrons orbit the atom's nucleus at certain distances. The
distance from the center of the nucleus is calculated by the
orbital radius = 0.053nm x
In this case, n is the level of elevtron orbit
(1, 2, 3, etc.). Therefore, the first orbit is 0.053nm from the center of the
nucleus. The second is 0.21nm (or 0.053 x 2^2). The third is 0.48nm. You
cannot have an electron orbit in between two of these levels - it just doesn't
happen. This is what we mean when we say that they orbots are
As an atom gains energy, its electrons can
move from a lower orbit to a higher one. We say that the atom is
excited. A common way that atoms become excited is by
absorption of light. Only certain wavelengths of energy can be
absorbed and stored in an atom, thus raising an electron to a higher
The reverse process, where an atoms can lose energy as a photon of
light, is called emission. In this case, an electron drops
from a higher orbit to a lower one.
Spectroscopy is the
process of capturing and analyzing the spectrum of electromagnetic radiation
(light) given off by an astronomical body. Because the wavelengths of light
absorbed and emitted depend on the kind of atoms absorbing and emitting them,
this can tell us about the bodies we are looking at.
If an electron in a
hydrogen atom drops from orbit 3 to orbit 2, it gives off a photon of light that
has a wavelength of 656nm - bright red light. If that same electron were to
drop from orbit 4 to orbit 2, the photon would have a wavelength of 486nm and be
a turquoise blue color. If we were to look at a spectrum of light given off by
excited H atoms, we would only see light at these two wavelengths. This
is hydrogen's emission spectrum - it shows the colors given off
On the other hand, if we were to shine some white light
through a cloud of hydrogen and look at the colors that came out the other side,
we'd see a dark band at the places in the spectrum that correspond to those two
wavelengths: 656nm and 486nm. This is hydrogen's absorption
Astronomers use light emitted from atoms, compounds,
or even reflected off of solid surfaces to tell them something about the distant
objects they study.
Types of Spectra
Continuous Spectrum - all colors present, brightness changes
smoothly with wavelength. Typical of solid or dense objects.
Emission-Line Spectrum - light is emitted at only a few
specific wavelengths - the rest is dark. Typically produced by hot, tenuous gas
- i.e.: flourescent bulbs, aurora, and interstellar gas clouds.
Absorption Spectrum (aka - Dark-Line Spectrum)
- Similar to a continuous spectrum, but with dark lines at certain wavelengths
which were absorbed. Typically produced when the light from a hot, dense object
passes through cooler gas and some of the wavelengths get absorbed.
When a star is moving towards us, the light waves get compressed - so the
wavelengths get shorter. We say the light is blueshifted.
When a star is moving aways form us, the light waves get stretched out - so
they get longer. We say the light is redshifted.
We can use redshift and blueshift to tell whether a star is moving towards or
away from us, and even to calculate how fast it is moving.
The Doppler Effect
Homework from the
- Read Chapter 3 (pg.
- Answer Review Questions #1-9 (pg.