Lecture 11: Electromagnetic Waves

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. 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 10 8 m/s

Electromagnetic Spectrum

Some Applications IR imaging

Black Light

Speed of Light (In Vacuum) Foucault's Experiment Fizeau’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 when astronomers saw the explosion in 1987, they were actually seeing the light that left the supernova years earlier. Supernova Explosion

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 v rel 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. S o is the avg intensity reaching the analyzer IMAX 3D

LCD Display