Transcript electromagnetic wave

Lecture 11:
Electromagnetic Waves
04-24-07
James Clark Maxwell and EM Waves
We saw Electric and Magnetic fields earlier.
Scottish physicist James Clerk Maxwell (1831–1879) showed that
these two fields fluctuating together can form a propagating
electromagnetic wave.
Maxwell’s Equations: implicitly required the existence of
electromagnetic waves traveling at the speed of light. He also
proposed a physical theory of ether.
1831-1879
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35
An electromagnetic wave is a transverse wave because the electric and magnetic fields are both
perpendicular to the direction in which the wave travels. an Electromagnetic wave, unlike a wave on
a string or a sound wave, does not require a medium in which to propagate. Electromagnetic
waves can travel through a vacuum or a material substance.
All electromagnetic waves move through a vacuum at the same speed, and the symbol c is used to
denote its value. This speed is called the speed of light in a vacuum and is 3 x 108 m/s
Electromagnetic Spectrum
Some Applications
IR imaging
Black Light
http://www.electricalfun.com/blacklight.htm
http://www.mosespendleton.com/
Speed of Light
Fizeau’s experiment
(In Vacuum)
Foucault's Experiment
Michleson modified the experiment for better accuracy. Placed the fixed and rotating
mirrors on Mt. San Antonio and Mt. Wilson in California, a distance of 35 km apart.
Looking back in time
Supernova Explosion
when astronomers saw the explosion in 1987, they were actually seeing the light
that left the supernova 175 000 years earlier.
Energy Carried by Electromagnetic Waves
In an electromagnetic wave propagating through a vacuum or air, the electric field
and the magnetic field carry equal amounts of energy per unit volume of space.
Doppler Effect for EM Waves
Electromagnetic waves also can exhibit a Doppler effect, but it differs from that for
sound waves for two reasons.
First, sound waves require a medium such as air in which to propagate. In the
Doppler effect for sound, it is the motion (of the source, the observer, and the
waves themselves) relative to this medium that is important. In the Doppler effect
for electromagnetic waves, motion relative to a medium plays no role, because
the waves do not require a medium in which to propagate. They can travel in a
vacuum.
Second, in the equations for the Doppler effect in Section 16.9, the speed of
sound plays an important role, and it depends on the reference frame relative to
which it is measured.
The speed at which they travel has the same value, whether it is measured
relative to a stationary observer or relative to one moving at a constant velocity.
Because vrel is the relative speed, it has no algebraic sign.
Example 6
Polarization
Transverse nature of EM oscillations leads to Polarization
Polarization
Transverse nature of EM oscillations leads to Polarization
Malus Law
S is the avg intensity of light leaving the analyzer.
So is the avg intensity reaching the analyzer
IMAX 3D
LCD Display