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Unformatted text preview: Physics 112 Test 2
Physics for Scientists and Engineers Dr. Clark
20 October 2005 Do all problems. Show all work and explain your reasoning to receive full credit. Do not
use formulas or integrator features of calculators. If you do not understand a question or
find it ambiguous, ask the monitor about it. A. Multiple Choice Problems: (8 points each) Circle the most correct answer. Box in
your second choice for potential partial credit. 1. The index of refraction of a typical dielectric can be described by the equation,
n = A  B/AZ, where A and B are positive constants for the particular dielectric and xi is the
wavelength of light. Based on this equation, the critical angle for yellow light is
the critical angle for red light, if the dielectric is located in air.
n , A~ e/ 2 )\ < x Q.
A a W,
@ greater than I E > E. aw. " ‘3 g ‘7'
b. less than  7‘ 3‘9: 6 38“.
'3 ‘ ‘3
c. the same as
d. none of the above because there is insufﬁcient information to determine
e. none of the above because there is no critical angle 2. A plasma (a gas of charged particles) is strongly dispersive to EM waves over a broad
range of radio frequencies. When a signal containing many such frequencies is incident
on the plasma in an experimental cell, it is observed that the higher frequencies reach the
opposite end of the cell before the lower frequency waves do. Which statement below
could be reasonably concluded from this observation? ’UK>VL. @The plasma refractive index decreases with increasing frequency. n > n
\ . ‘Y‘ b. The plasma refractive index increases with increasing frequency. c. The plasma refractive index does not seem to depend on frequency.
d. The higher frequency waves are propagating faster than the speed of light.
e. The lower frequency waves are being internally reﬂected. 3. In examples and homework problems we routinely used the equation relating wave
intensity and electric field amplitude for a plane wave and applied it to cases of spherical
waves (e. g. a radio antenna). Under what conditions is this legitimate? a. The same equation is valid for spherical waves everywhere. ® The same equation approximately valid for spherical waves far from the source, where
spherical waves behave approximately like plane waves.
c. The same equation is valid for spherical waves near the source, when we are far from
the source.
(1. It is never appropriate to treat spherical waves as plane waves, because the resulting
errors are always much larger than the magnitude of the value being calculated. 4. A real object is located at a distance 3f in front of a diver ’ng lens image is a. real and inverted. r, “3.3.._q_ .. .. _.....———
b. real and erect. L’WW . . .. c. virtual and inverted. 3%  @virtual and erect. r‘“ the focal length changed?
.L  l "
Ln 1 (\ \) (l ‘2"; \ a. Both focal lengths increase.
b. Both focal lengths decrease. © The focal length of the lens becomes greater than the focal length of the mirror.
d. The focal length of the lens becomes shorter than the focal length of the mirror.
e. The focal length of the mirror and lens remain unchanged. B. Short Calculation Problems: (8 points each) Show all work. 6. A spark gap consisting of two needle points charged to a potential difference will break
down when the voltage reaches 12 kVolts/cm. To what magnetic ﬁeld does this correspond? E : C B
‘1 NOSV g
'73’21" ,_ rt M0 Vs
”fl ' ————"’8 1.
3‘ 'b M 03"“) 3 “'10 M. 7. A laser beam is incident on a glass window pane at an angle of 50.0 degrees. Find the
distance (in cm) the beam travels inside the glass. Take the pane thickness d = 10.0 cm
and the index of refraction n = 1.50. 8. Microwaves of wavelength 3.00 cm are incident normally (i.e. perpendicular to the
wall surface) on a concrete wall 4.00 m thick. You measure the time it takes for the beam
to pass through the wall as 45.0 ns. What is the index of refraction of the concrete for these microwaves? q
4"” 0"“  o .o<a%€\><10'""ls r\=3:0°~ / a ' gm, ,,,,, “.9 ASHO‘ S  gﬁﬁleM‘S ‘” “’00“ 3.0 o 0 “Wks _ /
n ' 9.3ﬁﬁiO7M‘S : 337K104" ‘ 0 sM .~ ~ Z.
e ' i 4C? Mum u=<z= 1(H.zw~) ’L 10. You plan to go out in space for a day of solar sailing in a region of space where the
radiation intensity from a local star is 1000 Watt/m2. What is the momentum per m2 delivered to a 50% reﬂecting sail? K M‘ ‘k—ul
P ; /0°0"‘/7, 5’11”“ '2 ﬂ? : “‘vuglsmomm
9”“ m 9014.61 — “ts '
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— m‘J
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‘ c; ' Hw'w‘“
: '8’ K9 "/5
/000N/M = 333%“? _/ 3X/OGM‘S mks
But So 5.x... ahsmw was 90 ”W“ 7"
“ Z ('i) PC}: Pym1L §ﬂz~k 'm .5 Co l P 
1 ‘ 2 t3 3 04 My"); 11500 HO YKgm/S
= ‘iPu/mzs : " (333w . M2" ‘ “'5
”My ) C. Full Problems: (20 points each) 6) ’l: M r‘ M M
11. A myopic individual has an eye with a focal length of 3.00 cm. What is the required
strength of a corrective lens located 2.0 cm from the lens of the nearsighted person who
can see objects the clearest if they appear 15 cm from the corrective lens? Assume the real object is 50 cm from the corrective lens®Where is the final image formed?\Wl&is_ﬂ S the magnification of each lens and the system magniﬁcation? You must verify your Law?!)
answers with aproper ray diagram. _ M ,
WW mmwwmmww : = mm”?
Parker“? ‘l"+‘_ \ §o=\9m'Ur=\q“~ 6 So
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 2 ‘FZ 1’ \ ’7’ 12. A laser beam is incident on a 2.00 mm thick glass filter at normal incidence. The
filter scatters 70 % of the beam per mm of length. Write down an equation that
describes the intensity of light as a function of distance inside the filter. What fraction of
the incident light is transmitted through the ﬁlter? If the incident beam has 1015 photons / mm3 at a frequency of 5.00 x 1014 Hz, what in the amplitude of the
associated B field just before it enters the ﬁlter? ’/€X +2 /V" A/a Z
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