Light can be thought of as a WAVE...

Think about ocean waves:

Ocean Waves

= wavelength = distance between crests (or any two identical spots). The units are distance units like feet or meters.

f or = frequency = Number of times a boat goes up and down per unit time. The units are 1/(time), e.g. 1/second. Sometimes this is stated as cycles/second or Hertz (Hz).

v = speed of the wave. Units of distances/time.

The boat goes up and down with the waves. What about the water?

Q. Just what is traveling in the direction of the waves?

Energy and information. Waves are a disturbance traveling through a medium (although E-M radiation can travel in a vacuum).

There are other kinds of waves (ocean waves are sometimes called "gravity waves" or "surface waves")

So, what does this have to do with LIGHT?

Q. What is the speed of light in miles/hour?

c = 3 x 105  km
  x   0.62 miles
  = 186,000 miles/sec.
sec 1 km

186,000   miles
  x   60 sec
  x   60 min
  = 6.7 x 108 miles/hr.
sec 1 min 1 hour

Q. The Sun is 93,000,000 miles away. How long does it take for the light that leaves the Sun to reach the Earth?

Time =   9.3 x 107 miles
  = 500 sec x   1 min
  = 8.3 minutes.
186,000 miles/sec 60 sec

Q. What is a Light Year?

First, this is a unit of DISTANCE (not time). It is the distance light travels (in a vacuum) in 1 year.

Light travels:

186,000   miles
  x   60 sec
  x ...
sec 1 min

= 5.86 x 1012 miles/year (!)

Lookback Times: The disk of our Galaxy has a diameter of around 50,000 LY. The nearest big galaxy (Andromeda) is 2 million light years distant. So we see Andromeda as it looked 2 million years ago.

The most distant objects we have observed in astronomy have "look-back" times of over 8 billion years. The finite speed of light is very handy in that it allows us to look back in time...

More on Waves

There are only a few properties of light...

There is an alternative description for E-M radiation in which the light energy is carried in Photons (like little bullets of light) that have associated energy, wavelength and frequency. We will use these descriptions interchangably.