Electromagnetic Waves

 Concept and Nature of EM Waves  Frequency, Wavelength, Speed  Energy Transport  Doppler Effect  Polarization

Electromagnetic Waves Connect conducting rods to the terminals of an AC generator: Generator EMF produces current as charges separate Current produces magnetic field

Electromagnetic Waves Current reverses: magnetic field also reverses After current has reversed, charges are again separated with reverse polarity Electric field is then reversed

Electromagnetic Waves Magnitude and direction of electric and magnetic field vectors then travel away from the conducting rods. Traveling disturbance: wave.

Electromagnetic Waves The electric and magnetic field vectors oscillate in perpendicular planes. Both planes are perpendicular to the direction of the wave’s motion (transverse wave).

Electromagnetic Waves Just as the electric and magnetic fields require no material in which to exist, the electromagnetic wave needs no “medium.” It can travel in a vacuum, or in (some) materials.

Electromagnetic Waves James Clerk Maxwell 1831 – 1879 Scottish mathematician Established the theoretical basis for electromagnetic waves

Frequency, Wavelength, Speed Maxwell’s work in electrodynamics predicted:  the existence of electromagnetic waves  their transverse nature  their ability to travel without any material medium  their speed: (in vacuum; slower in materials)

Frequency, Wavelength, Speed Velocity, frequency, and wavelength are related in the same way as with other waves:

Frequency, Wavelength, Speed Electromagnetic waves are called by different names, and produced, handled, and detected by different technologies – depending on their frequency and wavelength.  radio waves  microwaves  infrared radiation  visible light  ultraviolet light  gamma waves

Energy Transport Like all waves, electromagnetic waves carry energy from one place to another. The time rate at which the energy passes a given location is power: The unit of power, as always, is the watt (W).

Energy Transport If an area A has a power P passing perpendicularly through it, we define a quantity intensity: Intensity can be expressed in terms of the electric and magnetic field peak magnitudes, individually: SI unit: W/m 2

Energy Transport If an element of area A intercepts an electromagnetic wave of intensity S, traveling in a direction that makes an angle  with the normal to the area element’s normal, the power within the area is

Energy Transport Energy per unit volume in a space traversed by an electromagnetic wave:

Doppler Effect Similar to sound waves: observed frequency depends on velocity of source and/or observer Different from sound waves:  No “medium” (depends only on relative source/observer velocity)  All observers, regardless of velocity, measure the same speed for light  Assumption: source/observer velocity small compared to the speed of light

Doppler Effect Governing equation: observed frequency source frequency relative source/observer velocity speed of light “+” means approaching; “ – “ means receding

Polarization The state of polarization of an electromagnetic wave refers to the orientation of the plane in which the electric field vector oscillates.

Polarization Some materials (polarizers) have a preferred direction for the electric field in the electromagnetic waves that they will transmit. An efficient polarizer transmits about half the randomly-polarized incident intensity.

Polarization If two polarizers are encountered in series, the transmitted intensity depends on the relative orientation of their transmission axes.

Polarization Etienne Louis Malus French artillery officer and engineer 1775 – 1812 Malus’ Law: