In astronomy class, we're going to talk about three different types of spectra
that are produced when light interacts with matter.
There are continuous spectra, which look like a rainbow spectrum where all the colors blend
together.
There are emission spectra, which look like
just bright lines on a dark background.
And then finally there is something called
an absorption spectrum, or an absorption line spectrum.
Sometimes it's called a dark line absorption spectrum.
And this is where it looks like there is a continuous background,
but certain colors have been removed and so you'll see dark lines in the spectrum.
Let's talk about how each of these are produced.
In physics the principles that govern how
different types of spectra are produced are called Kirchoff's Laws.
And one of these says that when atoms inside a hot, dense object are excited or energized,
they give off light of all the different colors and make a continuous spectrum.
And so it looks like a rainbow with all the colors blended together.
So hot, dense sources give off continuous spectra.
For example, the tungsten filaments inside an incandescent light bulb are hot and dense,
and thus give off continuous spectra.
The interiors inside of stars are hot and
dense, and they give off continuous spectra.
To produce an emission spectrum, then what
we have to To produce an emission spectrum what has to
occur is that the atoms inside a hot LOW density cloud of material or gas has to be energized.
And in that case only specific colors will be produced thus making an emission line spectrum
which consists of certain colors, certain lines, and we have a black background.
It looks like this.
And so we have a hot, low density gas.
For example, it could be
a nebula in space.
Well, that's only going to produce the specific
colors that the atoms in that cloud are sensitive to.
What's happening at the atomic scale is that the electrons in those atoms in the cloud
of gas have been energized, and they're dropping down levels and emitting
photons of light.
Invidividual atoms may be able to emit more
than one color of light, but they're not going to produce an entire
continuous spectrum.
And so what we'll see is an emission line
spectrum like we have here on the right.
Looks like brightly colored lines on a dark
background.
Finally, if we take the light that's produced
by a continuous source, and then pass it through a lower density and
cooler temperature cloud of material or cloud of gas, and then look at the light that emerges,
the spectrum will be an absorption line spectrum.
It looks like this.
Let's say that we start with the continous light coming from a hot, dense source, and
then pass it through a lower density and cooler collection of atoms.
Those atoms will absorb the colors they are sensitive to.
It's not going to be every color,
just some of them.
And so on the other side, if we take the emerging
light and pass it through a prism or a spectrograph, what we'll see is what appears to be a continous
background with certain colors removed.
An absorption line spectrum.
At the atomic level, what's happening is that all of that light, essentially white light,
passes through the cloud of gas, the atoms in that cloud of gas absorb certain
colors of light that those atoms are sensitive to.
They'll only absorb certain colors because they only have specific electron energy transitions
that the electrons are able to perform.
They can't do every single color, so you'll
only get certain colors absorbed.
All the others pass through that cloud of
gas, and so in the end what you get is that absorption line spectrum.
It turns out that stars always have absorption line spectra, and that's because even though
the interior is always hot and dense, as you go to the surface of the star, the
interior is surrounded by lower density, cooler material.
And so the continuous light that is produced inside of a star has to filter up through
the outer layers of the star, and thus the emerging light is an absorption
line spectrum of light.
