Think about ocean waves:
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")
Q. What is the speed of light in miles/hour?
|c = 3 x 105||km
||= 186,000 miles/sec.|
||= 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.
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...
There are only a few properties of light...
For light in a given medium (for example the vacuum of space), the speed is constant so Frequency and Wavelength are essentially different ways of describing the same thing.
|c = f x f =||c
Frequency (f) and wavelength are inversely proportional to one another - as one goes up, the other goes down to keep their product constant.
The energy of an E-M wave depends on the amplitude and the wavelength. For a given amplitude, shorter (higher f) waves carry more energy.
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.