Electromagnetic Waves Light! Review of Waves

Mondays 5:30 - 7:00 pm NSC Room 118

The cosmic speed limit (as Einstein would later establish). Nothing in the universe travels faster than light. It’s no coincidence that the speed of a propagating electromagnetic wave is the same as the speed of light. (We’ll firm up this connection later when we study electricity and magnetism…)

“Propagating electromagnetic waves” equal light waves. Light acts like a wave. The sun radiates energy from its surface throughout the solar system. That energy emerges in the form of electromagnetic radiation (light). Given that the sun is 1.5 X m away from Earth, how long does it take light from the sun’s surface to reach the Earth?

t = d / v = 1.5 X m / 3 X 10 8 m/s = 500 seconds or about 8 minutes! Most astronomical distances (outside the solar system) are so large that they are measured in lightyears, the distance light travels in a year. On lightyear = c t year = (3 X 10 8 m/s)(  X 10 7 sec) = 9.46 X m!!!!

The nearest star (Proxima Centauri) is about 4 lightyears away, which translates to about 38 X km! If Proxima Centauri disappeared today, we would not know about it until January 2003! When we look into the heavens, we are really seeing things as they were when the light rays left their sources. We are looking back in time!

…carry energy and momentum without permanently moving particles. What quantities characterize a wave? Amplitude Wavelength velocity Amplitude Wavelength velocity

Crest Trough One wavelength consists of exactly one crest and one trough. It is convenient to measure wavelengths from crest to crest, trough to trough, or 0 to 0. Crest Trough

Waves in Space... time As the wave propagates to the right, crests and troughs pass each point in succession. What do we see if we sit on the red line?

We see the crest of the wave, followed by a trough, followed by another crest, etc., etc., etc. So, if we plot the amplitude of the wave at the location of the red line as a function of time, we would see the following behavior... time

The period of a wave is the time that passes between the passage of two crests (or two troughs) at a given point in space. The period is related to the velocity and wavelength of the wave by the familiar expression d = v t period time

d = v t period (T) time In the case of a travelling wave, the distance traveled in one period is the velocity of the wave times the period of the wave. This quantity is, by definition, the wavelength. = v T