Light carries energy--we know this because light heats things up, it pushes with "radiation pressure" that forces the tails of comets outwards and stellar winds out from stars.

How does the energy get from place to place? Is it like a wave (like sound waves) or like a particle (a solid thing like a bullet?)

Both. We now understand light through the theory of quantum mechanics developed between 1900 and 1930.

For example, the colors we see on CDs, or rainbows, or the way if you put your fingers close together you'll see dark lines between them.

When the grooves on the CDs reflect light, the light that goes down into a deeper groove goes a longer distance. So when it comes back to meet the light that goes a shorter path, the two waves can be in phase or out of phase. Waves in phase add up together, but out of phase waves cancel and you don't see any light from them. Different colors have different wavelengths and if the extra distance bouncing off the deeper groove is a multiple of the light's wavelength, then it comes back in phase. Some colors have wavelengths that cause the waves to add up in phase when the bounce off the groove and other cancel out--that's why we see the colors.

The wavelength of the light we see is pretty short. If the tiny tiny grooves on a CD are as big as a half wavelength, so that some colors cancel out when they go the extra distance, that wavelength must be very small!

We measure the wavelength of light in nanometers (10^-9 meters) or sometimes in Angstroms (10^-10 meters. Blue light has a wavelength of about 400 nm, while red light has a wavelength of about 750 nm.

The theory is that light is a wave of electric and magnetic fields. You can create a kind of light using electrical circuits---these are radio waves, which were predicted first by the theory before they were made. You use an antenna to make radio waves or to detect them.

So depending on the wavelength there are all sorts of different kinds of light, not just the visible light that we see. Radio waves are one kind, which a much longer wavelength than visible light.

The wavelength of light and its frequency are related by the speed of light. Light is way fast. Its speed is 2.99792x10⁸ meters per second! The frequency of light is the number of times the peak of a wave will go past you per second. Frequency is measured in Hertz (abbreviated Hz), or cycles per second. The relationship between the wavelength of light and its frequency is

lambda f = c

where lambda is the wavelength and f is the frequency and c is the speed of light (remember E=m c²!)

Why believe this equation? Well, what is says is that f times per second, a wave will pass your location. Now each wave is of length lambda. So the total distance the wave is travelling per second in order to reach you is lambda times f. Speed is distance / time, or lambda times f divided by 1 second.

Brightness = (sigma) A T⁴

Here, sigma is a constant. If you use MKS units (measure all distances in Meters, masses in Kilograms, and time in Seconds), then the value of sigma is 5.7x10^-8. "A" is the surface area of the hot object. So something with more surface area is going to give off more light. But given equal surface areas, something hotter (higher temperature T measured in Kelvins) is going to give off more light, is going to be brighter.

You might be used to thinking of things that are "red-hot" as being very hot. Well, blue-hot is even hotter! That's why in a candle flame, the blue flame is closer to the wick, while the orange flame is further away, where it's not heated as much. That's also why the flame on an oven burner glows blue when it's very hot.

The mathematical form of Wien's Law is:

lambda_max=2.9x10⁶ nm / T_(K)

Here, T_(K) is just the temperature measured in Kelvins.

As an example, the Sun has a temperature (at its surface!) of 5,800 K. At what wavelength does its spectrum peak? (At what color is it brightest?) Using Wien's Law we find:

lambda_max=2.9x10⁶ nm / 5.8x10³

Now 2.9/5.8=0.5, so this gives us:

lambda_max=0.5x10³=500 nm

A wavelength of 500 nanometers is a yellowish green. But the Sun doesn't appear green! Well, the colors around 500 nm average it out, and some of the blue color is scattered in the air to make the blue sky--what's left over appears to us to be yellow.

Instead, in a fluorescent bulb, electrical sparks cause the gas inside to become ionized, so that electrons are kicked off from atoms. When the electrons recombine with the atoms, they can only go down through specific ladder rungs of energy. This is also part of the quantum mechanics theory. When an electron goes from a higher orbit to a lower one, it loses energy, and that energy is given off in the form of a photon (a particle of light.)

Yes, in quantum mechanics, light is both a wave and a particle. For example, you can use a Geiger counter to count individual photons of gamma rays. Gamma rays are just another form of electromagnetic radiation, just with a very short wavelength.

The energy of a single photon of light is related to the frequency of the light that it makes up. This is Planck's Law:

E = h f

This relates the energy E of the photon to the frequency f of the wave. It's impossible to visualize the photon being both a wave and a particle, so don't try too hard! The number h is a constant, and in MKS units it is equal to 6.626x10^-34. Don't waste your time memorizing this number! Just notice that it's a very very small number. Even though the frequency of visible light is pretty high, something like 10¹⁵ Hz, the energy in a single photon of visible light is miniscule.

So what does all this physics and light bulb stuff have to do with astronomy?

Well, whenever you look at some gas that's too thin to be a blackbody (so that the light coming from it hasn't acclimated itself to the average energy of heat motion), and is not in front of something brighter (where it might cause absorption), and this gas is either hot or being shone on by something else that's hot, then you'll see emission lines.

1. What something's made of: each atom has its own specific energy levels. So each atom has a "signature" list of emission lines. A thin gas made of that atom will give out a spectrum with only those specific wavelengths. So astronomers can tell what stars, galaxies, planets, and nebulas are made of by looking at their spectra. In fact, this is how the element Helium was discovered: in the spectrum of the Sun, before it was discovered in helium balloons!

   

   Above is a spectrum I used in my research. The horizontal axis shows the wavelength of light in a unit called "Angstroms"; 10 A are one nm. So the wavelength is between 110 and 170 nm, or in the ultraviolet region. We can't see this from the surface of the Earth. This spectrum was made by pointing the Hubble Space Telescope at a double-star neutron star system.

   You can see many emission lines in this spectrum. They are labelled by the element and the stage of ionization. N V is the 5th stage of ionization of the element Nitrogen, for example. There is also some continuum light and some absorption lines from gas in space between us and the star system.

   The emission lines are caused by X-rays from the neutron star shining on the star's outer layers--just like a fluorescent light is lit up by a spark to make emission lines.

2. Emission lines also tell you how something is moving towards or away from you. We can do this through the Doppler Effect.

   

   The Doppler Effect is familiar from every-day life from when you hear a train whistle or police car siren. When the siren comes towards you, the sound is higher pitched, but when it goes away from you, it's lower pitched.

   Something very similar happens with light. If something moves towards you, then its waves of light will come to you closer together, and it will appear to have a shorter wavelength. So you will see things moving towards you as blue-shifted. Likewise, something moving away from you will appear red-shifted. This is the Doppler Shift.

   The amount of the shift in wavelength is proportional to how fast the thing is moving. The equation for this is:

   Delta lambda / lambda₀ = v / c

   Delta Lambda is how much the wavelength has shifted. Lambda₀ is the original wavelength that was given off; it's the wavelength you would see if the thing wasn't moving. v is the speed the thing is moving and c is again the speed of light, 2.99792x10⁸ m/s.