Lecture6 (Chap 6)

7/28/2019 Lecture6 (Chap 6)

1/23

Electromagnetic Waves

The Nature of Electromagnetic Waves

The Energy carried by Electromagnetic Waves

The Doppler Effect and Electromagnetic Waves

Polarization


2/23

24.1The Nature of Electrom agnetic Waves

Two straight wires connected tothe terminals of an AC generator

can create an electromagnetic

wave.

Only the electric wave traveling to the

right is shown here.


3/23


The current used to generate

the electric wave creates a

magnetic field.


4/23


This picture shows the wave of the radiation field far from the

antenna.

The speed of an electromagnetic wave in a vacuum is:

sm1000.3 8c


5/23


A radio wave can be detected with a receiving antenna wire

that is parallel to the electric field.


6/23


With a receiving antenna in the form of a loop, the magnetic

field of a radio wave can be detected.


7/23

24.2The Electrom agnet ic Spectrum

Like all waves, electromagnetic waves have a wavelength andfrequency, related by:

fc


8/23

24.2The Electrom agnet ic Spectrum

Examp le 1The Wavelength of Visible Light

Find the range in wavelengths for visible light in the frequency rangebetween 4.0x1014Hz and 7.9x1014Hz.

nm750m105.7Hz104.0

sm1000.3 7

14

8

f

c

nm380m108.3Hz107.9

sm1000.3 714

8

f

c


9/23

24.3The Speed of Ligh t

The speed of l ig ht

in a vacuum

sm458792299c


10/23

24.4The Energy Carr ied by Electrom agnet ic Waves

Electromagnetic waves, like water waves, carry energy.


11/23


The total energy d ensity carr ied by an electromagn et ic

wave

22

2

1

2

1

Volume

energyTotalBEu

o

o


12/23


cutA

uctA

AtA

PS

energyTotal


13/23

24.5The Doppler Effect and Electromagnet ic Waves

Electromagnetic waves also can exhibit a Dopper effect, but it

differs for two reasons:

a) Sound waves require a medium, whereas electromagnetic

waves do not.

b) For sound, it is the motion relative to the medium that is important.

For electromagnetic waves, only the relative motion of the sourceand observer is important.

cvc

v

ff so

rel

rel

if1


14/23

Exercise

Police use radar guns and the Doppler effect to catchspeeders. A radar gun emits an electromagnetic wavethat reflects from the oncoming car as shown in thefigure below. The reflected wave returns to the police carwith a frequency (measured by on-board equipment) that

is different from the emitted frequency. Suppose that apolice car is moving to the right at 25 m/s, while thespeeder is coming up from behind at a speed of 37 m/s,both speeds being with respect to the ground. Assumethat the electromagnetic wave emitted by the radar gun

has a frequency of 8.0 x 109 Hz. Find the differencebetween the frequency of the wave that returns to thepolice car after reflecting from the speeders car and theoriginal frequency emitted by the police car.

24 6 P l i t i


15/23

24.6Polarizat ion

Linearly polarized wave

on a rope.

POLARIZED ELECTROMAGNETIC WAVES

24 6 P l i t i


16/23

24.6Polarizat ion

In polarized light, the electric field

fluctuates along a single direction.

24 6 P l i t i


17/23

24.6Polarizat ion

Polarized light may be produced from unpolarized light with

the aid of polarizing material.

24 6 P l i t i


18/23

24.6Polarizat ion

MALUS LAW

2cosoSS

intensity before

analyzerintensity after

analyzer

24 6 Polarizat ion


19/23

24.6Polarizat ion

Example 7Using Polarizers and Analyzers

What value of should be used so the average intensity of the polarized

light reaching the photocell is one-tenth the average intensity of the

unpolarized light?

24 6 Polarizat ion


20/23

24.6Polarizat ion

2

2

1

10

1

cosoo SS

2

5

1 cos

51

cos

4.63

24 6 Polarizat ion


21/23

24.6Polarizat ion

When Polaroid sunglasses are

crossed, the intensity of the

transmitted light is reduced to

zero.

24 6 Polarizat ion


22/23

24.6Polarizat ion

IMAX movie projector

24 6 Polarizat ion


23/23

24.6Polarizat ion

THE OCCURANCE OF POLARIZED LIGHT IN NATURE

Lecture6 (Chap 6)

Documents

Transcript of Lecture6 (Chap 6)