This print-out should have 17 questions.Multiple-choice questions may continue onthe next column or page – find all choicesbefore answering.001 10.0 pointsConsider the electromagnetic wave pattern asshown in the figure below.EBWhat is the direction in which the wave istraveling?1. from-left-to-right. correct2. undetermined, since the figure shows astanding wave.3. from-right-to-left.Explanation:
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This print-out should have 17 questions.

Multiple-choice questions may continue on

the next column or page – find all choices

before answering.

001 10.0 points

Consider the electromagnetic wave pattern as

shown in the figure below.

E

B

What is the direction in which the wave is

traveling?

1. from-left-to-right. correct

2. undetermined, since the figure shows a

standing wave.

3. from-right-to-left.

Explanation:

The direction of traveling is the direction of

E~ × B~ ; i.e., from-left-to-right.

002 10.0 points

A point light source delivers a power P. It

radiates light isotropically (equally in all directions). A mirror of area A is placed at

point B that is at distance r from the source.

The mirror’s surface is perpendicular to the

vector from the source to point B. Make the

approximation that all the light hitting the

mirror is perpendicular to it. The mirror is a

totally reflecting surface.

B

r

Find the total force on the mirror.

1. F = P

c

2. F =

2 π r2 P

c A2

3. F = A2 P

2 c π r2

4. F = A P

c 2 π r2 correct

5. F = 4 π r2 P

c A

6. F = P

c 2 π r

7. F = 2 P

c

8. F = π r2 P c

9. F =

2 P

c π r2

10. F =

2 A P

c r2

Explanation:

Since the mirror is small enough (or far

enough away) that we may approximate the

incoming light as perpendicular, the force is

simply

F = p A ,

where p is the radiation pressure. The mirror

is totally reflecting, so

p =

2 I

c

.

Intensity is defined by

I = Power hitting the mirror

Area of mirror A .

I =

P 4πr A 2

A =

P

4 π r2 ,

leading to a force of

F = p A = 2 I A

c

=

A P

c 2 π r2

on the mirror

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