Electromagnetic WavesLight!

Review of Waves

Mondays

5:30 - 7:00 pmNSC Room 118

c m s 2 99792 108. /

The cosmic speed limit (as Einsteinwould later establish). Nothing inthe universe travels faster than light.

It’s no coincidence that the speed of apropagating electromagnetic wave is thesame as the speed of light. (We’ll firm upthis connection later when we studyelectricity and magnetism…)

“Propagating electromagnetic waves”equal light waves. Light acts like a wave.

The sun radiates energy from its surfacethroughout the solar system. That energyemerges in the form of electromagneticradiation (light).

Given that the sun is 1.5 X 1011 m away fromEarth, how long does it take light from thesun’s surface to reach the Earth?

t = d / v = 1.5 X 1011 m / 3 X 108m/s = 500 seconds or about 8 minutes!

Most astronomical distances (outside the solarsystem) are so large that they are measuredin lightyears, the distance light travels in a year.

On lightyear = c tyear

= (3 X 108 m/s)( X 107 sec) = 9.46 X 1015 m!!!!

The nearest star (Proxima Centauri) is about4 lightyears away, which translates to about 38 X 1012 km!

If Proxima Centauri disappearedtoday, we would not know aboutit until January 2003!

When we look into the heavens, we are reallyseeing things as they were when the light raysleft their sources. We are looking back in time!

…carry energy andmomentum withoutpermanently movingparticles.

What quantitiescharacterizea wave?

Amplitude

Wavelength

velocity

Amplitude

Wavelength

velocity

Crest

Trough

One wavelength consists of exactly onecrest and one trough. It is convenientto measure wavelengths from crest tocrest, trough to trough, or 0 to 0.

Crest

Trough

Waves inSpace...

tim

e

As the wavepropagatesto the right,crests andtroughs passeach point insuccession.

What do wesee if we siton the redline?

We see the crest of the wave, followedby a trough, followed by another crest,etc., etc., etc.

So, if we plot the amplitude of the waveat the location of the red line as a functionof time, we would see the following behavior...

time

The period of a wave is the time that passesbetween the passage of two crests (or twotroughs) at a given point in space.

The period is related to the velocity andwavelength of the wave by the familiarexpression

d = v t

period

time

d = v tperiod (T)

time

In the case of a travelling wave, thedistance traveled in one period isthe velocity of the wave times the periodof the wave. This quantity is, by definition,the wavelength.

= v T