23 Electromagnetic Waves

Principle

Faraday’s law: time-varying B-field creates E-field (emf)Maxwell: time-varying E-field generates B-field (emf)

EM waves were PREDICTED by J.C. Maxwell in 1865. Light is part of the EM wave

Heitz first detected the EM waves in 1887 after Maxwell died (1879).

Another great prediction(s) was made by Einstein from his General Relativity.

James C MaxwellBritish Scientist(1831 – 1879)

Heinrich R HertzGerman physicist(1857 –1894)

Properties of EM Waves

1. Transverse Wave; Both E & B perpendicular to propagation, k. E, B and k follow the right-hand rule

2. EM waves can be created and travel in vacuum (no medium required) with unchanging speed c

3. E & B are in phase and E= cB

(in vacuum)

smc /1000.31 8

00

v = f

EM Spectrum

Images Taken in Different EM Wave Regions

Sinusoidal Waves

kxtAx

T

tAtxy

sin2sin),(

Energy in EM Waves

cutA

tuAc

tA

Vu

t

U

AS

uuEu

Ec

EB

BEuuu

BE

BE

1

)(

2

1

2

1

20

00

20

20

S: energy per unit area per unit time or power per unit area and the average value, Sav called intensity, I.

Radiation Pressure

c

I

t

p

A

c

S

c

u

Vc

U

V

pc

Umcp

1 : Pressure Radiation

2

Nature of Light: Wave-Particle Duality

Wave Fronts

Geometric optics &Physical Optics

Rays & Waves

Reflection & Refraction

Index of Refraction & Snell’s LawNormal

Incident

Reflection

Refraction

a r

b

ra

bbaa nnv

cn

sinsin

Angles respect to normal lineAll the rays in same plane

nanb

Experience: Driving & Reflections

Value of Refractive Index

Air 1.0003

Total Internal Reflection

ab

ab n

n sinsin

Critical angle: when b = 90o,

a = c = sin-1(na/nb)

90

c

Dispersion: n= n()

Polarization

Linear polarization

Circular polarization (left)

Mirage, Blue Sky, & Traffic Lights

Summary: EM Waves, Speed & Spectrum

Sinusoidal Waves and Energy

Nature of Light

Reflection & Refraction

Total Internal Reflection:

Reflection & Refraction: Animations

Animation of Refraction (rays)

Animation of Refraction (waves)

Dispersion

Polarization

HomeworkCh 23 Problems: 3, 6, 13, 20, 26, 38, 45, and 49

Test III Score Distribution

Correct Points

Nu

mb

er of Stu

den

ts

Your grade =points/16x100%