Electromagnetic spectrum Frequency of visible light 4 3

Electromagnetic spectrum. Frequency of visible light: 4. 3× 1014 to 7. 5× 1014 Hz

Why a single wave phenomenon is called so many names? EM waves of different frequencies/lengths originate from different sources and interact with matter essentially differently.

Electromagnetic Waves http: //www. phy. ntnu. edu. tw/java/em. Wave. html http: //www. amanogawa. com/archive/Plane. Wave-2. html http: //www. walter-fendt. de/ph 14 e/emwave. htm http: //www. educypedia. be/electronics/javawaves. htm

Electromagnetic Waves • The E and B fields are perpendicular to each other • Both fields are perpendicular to the direction of motion – Therefore, EM waves are transverse waves • Propagate with the speed of light, which does not depend on either frequency or amplitude; • Speed of light in vacuum – a fundamental constant, c = 3 108 m/s In every point in space, at each moment of time

Polarization is a property of electromagnetic waves that specifies the direction of electric field. Why do we pick the electric field out of the two? Aren’t they on an equal footing? They are on an equal footing, but it is mostly the electric field, which interacts with matter – atoms, electrons, nuclei…

EM waves from a TV tower are perfectly polarized – the Electric field has a very well defined direction, which stays always the same. In contrast, the light coming from the Sun or from a light bulb is unpolarized. What does it mean unpolarized? Does not the electric field have some direction? It certainly does at every instant. BUT this direction does not stay constant and changes very rapidly and randomly. So, averaging over any reasonable time interval you do not find any particular polarization! polarized unpolarized The frequency of light is about 5× 1014 Hz, which means 5× 1014 wave crests per second. If the polarization changes once every 500 crests it will still be 1012 times per second. Too fast for us to detect!

Any way to make a polarized wave (light) out of unpolarized wave? Yes, but it is going to cost us some intensity loss… (No free meals…) We can use a polarizer - a piece of material, whose molecular or crystal structure has a preferred direction called the transmission axis. A polarizer “decomposes” the wave into a “proper” component with the electric field, , parallel to the transmission axis, which passes through, and a “wrong” component with totally absorbed. perpendicular to the transmission axis, which gets

The magnitude of the proper component of the electric field: z Intensity of the wave is proportional to the square of the amplitude transmission axis Law of Malus In an unpolarized wave, the angle q is changing randomly. Therefore, after passing through a polarizer the average intensity is The light gets polarized, but we loose 1/2 of its intensity. . .

If the axis of a polarizer is set at q = 90° to the axis of polarization no light is passing through! A system of two crossed polarizers never lets any light through. Whatever passes through the first one is blocked by the second.

What happens to the intensity, S, and direction of polarization of unpolarized light upon passing trough three polarizers shown here? E E E S 1 S 2 S = S 3 Without the second polarizer http: //www. colorado. edu/physics/Physics. Initiative/Physics 2000/applets/lens. html

EM Waves by an Antenna • Two rods are connected to an ac source, charges oscillate between the rods (a) • As oscillations continue, the rods become less charged, the field near the charges decreases and the field produced at t = 0 moves away from the rod (b) • The charges and field reverse (c) • The oscillations continue (d)

Electromagnetic Waves Produced by an Antenna • When a charged particle undergoes an acceleration, it radiates energy – If currents in an AC circuit change rapidly, some energy is lost in the form of EM waves – EM waves are radiated by any circuit carrying alternating current • An alternating voltage applied to the wires of an antenna forces the electric charge in the antenna to oscillate