Howard Community College Summer Astronomy 2016: Light

Big Distances

Consider light as a means of measurement. The speed of light (in a vacuum) is:

c = 3E8 m/s

(Which is around 186,000 miles per second or 670 million miles per hour, or nearly 7 times around the Earth's equator in one second.)

Since v = d/t, we see that d = v t.

Using light's speed, we can define a light-second:

1 LS = 3E8 m = 300,000,000 meters (or about 186,000 miles)

Similarly, a light year:

1 LY = 9.4607 E12 km (or nearly 6 trillion miles)

WAVES!

Let's talk some more about "light." What is it?

Consider first a "wave" -

Energy can, as it turns out, travel in waves. In fact, you can think of a wave as a traveling disturbance, capable of carrying energy with it. For example, light "waves" can have energy - like solar energy. Ocean waves can certainly carry energy.

There are several wave characteristics (applicable to most conventional waves) that are useful to know:

amplitude - the "height" of the wave, from equilibrium (or direction axis of travel) to maximum position above or below

crest - peak (or highest point) of a wave

trough - valley (or lowest point) of a wave

wavelength (lambda - see picture 2 above) - the length of a complete wave, measured from crest to crest or trough to trough (or distance between any two points that are in phase - see picture 2 above). Measured in meters (or any units of length).

frequency (f) - literally, the number of complete waves per second. The unit is the cycle per second, usually called: hertz (Hz)

wave speed (v) - the rate at which the wave travels. Same as regular speed/velocity, and measured in units of m/s (or any unit of velocity). It can be calculated using a simple expression:

There are 2 primary categories of waves:

Mechanical – these require a medium (e.g., sound, guitar strings, water, etc.)

Electromagnetic – these do NOT require a medium and, in fact, travel fastest where is there is nothing in the way (a vacuum). All e/m waves travel at the same speed in a vacuum (c, the speed of light):

c = 3 x 10^8 m/s

We care primarily about the electromagnetic (e/m) waves:

General breakdown of e/m waves from low frequency (and long wavelength) to high frequency (and short wavelength):

Radio

Microwave

IR (infrared)

Visible (ROYGBV)

UV (ultraviolet)

X-rays

Gamma rays

In detail, particularly the last image:

http://hyperphysics.phy-astr.gsu.edu/hbase/ems1.html