The energy carried by light can interact with matter in four general ways.
  1. Emission--matter can emit light. When you turn on a lamp, electricity flowing through the filament of the light bulb heats it to a temperature at which it emits visible light.
  2. Absorption--matter can absorb light. If you place your hand near a lit light bulb, your hand absorbs some of the light, and this absorbed energy makes your hand warmer.
  3. Transmission--some forms of matter, such as glass or air, transmit light; that is, they allow light to pass through them.
  4. Reflection--matter can reflect light. A mirror reflects light, similar to the way a rubber ball bounces off a hard surface.

In this lab we will be dealing with the first two types of interactions.

The universe is filled with a variety of wispy, tenuous gases, including clouds of gas between the stars and the thin atmospheres of stars and planets. Gases can absorb light; for example, ozone in the Earth's atmosphere absorbs ultraviolet light from space preventing it from reaching the ground. Gases can also emit light, which is what makes interstellar clouds glow so beautifully. There are a few pictures of these glowing nebulas up on the walls in the lab.

How do gases absorb and emit light? Most of the gas we see in the universe is hydrogen. Hydrogen is the simplest atom that exists. It has a nucleus of one proton and it has one electron `orbiting'  around the nucleus (see animation below). Orbiting is in quotes because we now know that electrons don't simply orbit the nucleus like planets orbit the Sun, although it was once thought that they did. However, it is still a good model to get an idea how the absorption/emission process works.

There is a particular `orbit' an electron can have where it is as close to the nucleus as it can get. We call this orbit the ground state of the electron and here the electron has the least amount of energy it can have. The electron, although it can't get any closer to the nucleus, can get to an orbit that is further away. The only way it can get to one of these `excited' states, is to absorb some energy that gives it enough oomph to jump up to one of the higher energy levels. The energy that it absorbs is the energy of light--a photon. When the electron drops back down to the ground state the extra energy is released in the form of a photon.
 

The discharge lamps that your instructor will show emit light because the current of electrons from the wall plug excite the electrons of the gas in the tube. Below is an animation of how the blue line of hydrogen is made.  The electron is excited into the fourth state by a collision with another electron, then it goes back down to the second state and emits the turquoise photon.  Then it quickly returns to the ground state, emitting a UV photon which our eyes are not sensitive to.  Other lines come from other transitions.  Each element has a different set of energy levels, so it absorbs and emits different photons and thus has different lines.
The electrons of atoms can gain and lose energy in other ways, too. Below there are two animations. One shows hydrogen's electron gaining energy from a collision. The other shows hydrogen's electron gaining energy from a photon that was emitted from another hydrogen atom.
Now one very important thing about all this absorbing and emitting and jumping about between energy levels is that there are only very particular energy levels or orbits that an electron can have and there are only very particular photons that an electron can absorb or emit. In order for an electron to switch energies it must absorb or emit an amount of energy exactly equal to the energy it needs to make the switch. A photon with too much or too little energy cannot be used in this process.

This is kind of an odd idea. Imagine if you owned a car that would jump from going 5 miles per hour to 20 miles per hour without gradually passing through a speed of 10 miles per hour. But think about it in terms of climbing a ladder. You can't stand between two rungs on a ladder, you are either on one step or another, and nowhere in between.

A final important process is ionization.  This is when the electron gains so much energy from a photon or a collision that it leaves the atom altogether.