Spectral Lines 

If we look at the spectrum of the Sun, we don't just see continuous blackbody radiation -- we see dark lines as well.

in 1860, Kirchhoff and Bunsen had been studying the spectral properties of matter and noted that some of the dark lines in the Sun were at the exact same wavelength as bright lines produced when sodium was heated up -- there must be sodium in the Sun.

Furthermore, they also published what are now known as Kirchhoff's Laws:

What is going on here?

To understand this, we need two concepts: photons and atomic structure


Photons

We have talked about how light is an electromagnetic wave. It can be characterized by wavelength and frequency. Different wavelengths correspond to different colors, and ultimately different types of radiation (UV, IR, radio, X-rays, etc).

Light can also be characterized by discrete particles called photons -- hence the wave-particle duality of light.

How much energy is contained in a single photon?

How fast does it move? How much mass does it have?

How was this discovered? Via the photoelectric effect:

It had been noticed that when light shines on a metal surface, electrons are ejected from the surface with a characteristic kinetic energy. If you increase the brightness of the light, you get more ejected electrons, but not at higher kinetic energies.

So even though you are bathing the metal in more energy, the electrons coming off have the same maximum energy.

Einstein explained this by suggesting light was made up of discrete particles -- photons -- that had a discrete amount of energy. More photons could eject more electrons, but not at higher speeds. This is what Einstein won the 1921 Nobel Prize for, not his work on special and general relativity.

Example: How much energy does a single blue (4000 Angstrom) photon have? Here is a quick tip to remember: hc=12400 eV A. So one 4000 A photon has 12400/4000=3.1 eV of energy.


Atomic Structure

At atom is made of a nucleus (protons and neutrons) with electrons "orbiting" around it. Quantum mechanics says that these electroncs cannot orbit with any energy they like, but must live at discrete, well-defined energy levels.

Consider the hydrogen atom - 1 electron in orbit around 1 proton. Simple!

The allowed energy levels in the hydrogen atom are given by
Think of this visually, in terms of orbits (this is not really correct, but it is a useful analogy...)

Now, what would happen if an electron moved from one level to another? The energy of the atom would change! How could this happen?
 

In the hydrogen atom, transitions to/from the n=2 level involve energies which correspond to optical photons. These transitions were the first discovered, and are called the Balmer series.
 
Transition
Name
Wavelength
n=3 to/from n=2
Halpha
6563 A
n=4 to/from n=2
Hbeta
4861 A
n=5 to/from n=2
Hgamma
4340 A
n=6 to/from n=2
Hdelta
4102 A
The first four Balmer lines

Other series include

Here is an energy level diagram for the hydrogen atom:
 
Enough nonsense -- show us a real spectrum! Okay, here is a spectrum of a hot, blue star called an A star.

(courtesy Diamond Dave Silva)

Wow! Look at those Balmer lines!