Let There Be LightLet There Be Light13.313.3

Where are we?

• Exam #3 on Monday Morning– Watch for another of those

Webby Things

• Today, we continue our study of optics and continue working with the optics kit.

• Next week … optics marathon.

Start readingchapter 24after the test

Which one of the following statements concerning electromagnetic waves is false?

A) Electromagnetic waves are longitudinal waves.B) Electromagnetic waves transfer energy through space.C) The existence of electromagnetic waves was predicted by Maxwell.D) Electromagnetic waves can propagate through a material substance.E) Electromagnetic waves do not require a physical medium for propagation.

The electric field E of an electromagnetic wave traveling the positive x direction is illustrated in the figure. This is the wave of the radiation field of an antenna. What are the direction and the phase relative to the electric field of the magnetic field at a point where the electric field is in the negative y direction?Note: The wave is shown in a region of space that is a large distance from its source.

A) +y direction, in phaseB) –z direction, 90° out of phaseC) +z direction, 90° out of phaseD)–z direction, in phaseE)+z direction, in phase

Think about the diagram we used yesterday and howthe charge moves to produce the wave in the first place!

Glare tends to be

A. un-polarized lightB. light polarized verticallyC. light polarized horizontallyD. the polarization depends on the

conductivity of the material causing the glare

Which one of the following colors of visible light has the highest frequency?

A) yellowB) redC) greenD) blueE) violet

Like all waves, electromagnetic waves have a wavelength and frequency, related by:

fc

The speed of lightin a vacuum

sm 458792299c

sm1000.3AmT104mNC1085.8

11 8

72212

oo

c

Maxwell’s prediction of the speed of light

The total energy density carried by an electromagnetic wave

22

2

1

2

1

Volume

energy TotalBEu

oo

Must contain moving energy

1 Total energyP Energy uctAS cu

A time Area t A tA

Maxwell’s Equations show that the energy density of the magnetic field and the electric field in an electromagnetic wave are the same.

Consequently2 2

00

2 20

0

0 0

1 1 (Peak Values)

2 2

1

since

1

get

E=cB

E E2 2

rms rms

u E B

so

u E B

c

we

E E

In polarized light, the electric fieldfluctuates along a single direction.

Polarized light may be produced from unpolarized light withthe aid of polarizing material.

IMPORTANT STUFF HERE

2cosoSS

intensity beforeanalyzer

intensity afteranalyzer

2 2

2 2

2

2

sin cos 1

sin cos

2 cos 1

1cos

2

Example 7 Using Polarizers and Analyzers

What value of θ should be used so the average intensity of the polarizedlight reaching the photocell is one-tenth the average intensity of theunpolarized light?

S0

S0/2

221

101 cosoo SS

251 cos

51cos

4.63

Conceptual ClickerSuppose that a third piece of polarizing material is inserted between the polarizer and analyzer. Does light now reach thephotocell?

A YesB NoC More info required.

Complete the last one first.

Prepare to Discuss These Problems After The Experiment:

1.A friend is standing 2 m in front of a plane mirror. You are standing 3 m directly behind your friend.

2.How far is your image behind the mirror?

3.What is the distance between you and the image of your friend?

4.Suppose that you are walking perpendicularly with a velocity of +0.90 m/s toward a stationary plane mirror. What is the velocity of your image relative to you? The direction in which you walk is positive direction.

I Feel Horrible!