terry (ect328) – homework 38 – Turner – (59130)
1
This
printout
should
have
10
questions.
Multiplechoice questions may continue on
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before answering.
001
10.0 points
A 1 mW laser beam is reflected back upon
itself by a mirror.
Calculate the force on the mirror.
The
speed of light is 2
.
99792
×
10
8
m
/
s.
Correct answer: 6
.
67128
×
10
−
9
N.
Explanation:
Let :
P
= 1 mW = 0
.
001 W
and
c
= 2
.
99792
×
10
8
m
/
s
.
The intensity of the laser is
P
A
with
P
the
power and
A
the area of the beam.
The
average momentum per unit area per unit
time (force per unit area) transferred to the
wall is
Δ
p
Δ
t A
=
F
A
=
2
I
c
=
2
P
A c
,
so
F
=
2
P
c
=
2 (0
.
001 W)
2
.
99792
×
10
8
m
/
s
=
6
.
67128
×
10
−
9
N
.
002
10.0 points
At what distance from a 68 W electromag
netic wave point source (like a light bulb) is
the amplitude of the electric field 22 V
/
m?
μ
o
c
= 376
.
991 Ω
.
Correct answer: 2
.
9034 m.
Explanation:
Let :
P
= 68 W
,
E
max
= 22 V
/
m
,
and
μ
0
c
= 376
.
991 Ω
.
The wave intensity (power per unit area) is
given by
I
=
S
av
=
E
2
max
2 (
μ
o
c
)
=
(22 V
/
m)
2
2 (376
.
991 Ω)
= 0
.
641925 W
/
m
2
.
The constant
μ
o
c
(impedance of free space)
turns out to be 120
π
Ω.
You should verify that (V
/
m)
2
Ω = W
/
m
2
.
With this we can find the intensity, which
multiplied by the area of a circle of radius
R
must gives us the source’s power
P
, therefore
4
π R
2
I
=
P
= 68 W
.
Solving for the radius we get,
R
=
radicalbigg
P
4
π I
=
radicalBigg
68 W
4
π
(0
.
641925 W
/
m
2
)
=
2
.
9034 m
.
003
10.0 points
A light wave passes from air into water.
Which of the following is true?
1.
Its frequency remains unchanged, but its
wavelength increases.
2.
None of these.
3.
Both its wavelength and its frequency
remain unchanged.
4.
Its wavelength remains unchanged, but
its frequency decreases.
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 Fall '09
 Turner
 Light, Snell's Law, Correct Answer, critical angle

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