### Fall 2009 Physics 102 Hour Exam 3(26 questions)

The grading button and a description of the scoring criteria are at the bottom of this page. Basic questions are marked by a single star *. More difficult questions are marked by two stars **. The most challenging questions are marked by three stars ***.

True-false questions are worth 2 points each, three-choice multiple choice questions are worth 3 points each, five-choice multiple choice questions are worth 6 points each. The maximum possible score is 131. The exam period was 90 minutes; the mean score was 100.7; the median was 103. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

• A reminder about prefixes: p (pico) = 10-12; n (nano) = 10-9; μ (micro) = 10-6; m (milli) = 10-3; k (kilo) = 10+3; M or Meg (mega) = 10+6; G or Gig (giga) = 10+9.

### QUESTION 1**

A thin film of oil (index of refraction 1.52) is poured on top of water (index of refraction 1.33). The oil is illuminated from the air above it by monochromatic light.

What smallest thickness t of oil would cause orange light (λ = 590 nm) to be most strongly reflected?

(a)   97 nm
(b)   110 nm
(c)   194 nm
(d)   210 nm
(e)   220 nm

### QUESTION 2*

This and the next question pertain to the following situation.

A double-slit experiment is set up as shown in the figure below. Light from a laser illuminates a pair of slits etched in an opaque barrier. The spacing between the slits is 3 × 10-4 m; the distance from the barrier containing the slits to a viewing screen is 3 m. Because of diffraction, a pattern of light and dark spots is seen on the viewing screen. The distance from the central bright spot to the first bright spots on either side is 4 × 10-3 m.

What is the wavelength of the laser light illuminating the pair of slits?

(a)   200 nm
(b)   400 nm
(c)   500 nm
(d)   1.22 × 103 nm
(e)   1.07 × 104 nm

### QUESTION 3**

If the separation between the slits is increased by 50%, the spacing between the bright spots on the viewing screen will

(a)   increase by 100%.
(b)   increase by 50%.
(c)   remain unchanged.
(d)   decrease by 33%.
(e)   decrease by 50%.

### QUESTION 4*

This and the next question pertain to the following situation.

A two-lens device is made from a convex lens with focal length f1 = 10 cm and a concave lens with focal length f2 = -20 cm as shown in the figure. A candle is placed 15 cm to the left of the convex lens. The image of the candle formed by the first lens serves as an object for the second lens which, in turn forms an image from this object. The lenses are 60 cm apart.

What is the position of the image formed by the concave lens?

(a)   45 cm to the right of the concave lens
(b)   40 cm to the left of the concave lens
(c)   20 cm to the left of the concave lens
(d)   15 cm to the right of the concave lens
(e)   12 cm to the left of the concave lens

### QUESTION 5**

The concave lens makes an image of the lit candle. In this image,

(a)   the candle's base is closer to the ceiling than is its flame -- the image is real.
(b)   the candle's base is closer to the ceiling than is its flame -- the image is virtual.
(c)   the candle's flame is closer to the ceiling than is its base -- the image is real.
(d)   the candle's flame is closer to the ceiling than is its base -- the image is virtual.
(e)   neither the flame nor the base is closer to the ceiling since the image is a point.

### QUESTION 6**

A source sends out two coherent beams of electromagnetic waves as indicated. A mirror is placed at a height h = 1 m and distance L/2 so that the two beams meet at point X a distance L = 8 m from the source. Assume no phase shift from reflection at the mirror.

If there was complete constructive interference at X, what is a possible frequency of the light?

(a)   0 Hz
(b)   0.25 Hz
(c)   0.375 Hz
(d)   4 × 108 Hz
(e)   1.2 × 109 Hz

### QUESTION 7**

Unpolarized light passes through three polarizers with initial intensity I0. The first polarizer is polarized vertically, the second polarizer is polarized with 45° to the vertical, and the third is polarized 75° to the vertical.

What is the final intensity of the light?

(a)   0.017 I0
(b)   0.033 I0
(c)   0.065 I0
(d)   0.188 I0
(e)   0.408 I0

### QUESTION 8*

Three polarizers in a row have polarization angles of θ, 2θ and 3θ respectively with respect to the z-axis. If an incident light wave that is z-polarized with intensity I0 passes through all of the polarizers, what is the angle θ if the final intensity is I0 / 26 ?

(a)   0°
(b)   30°
(c)   45°
(d)   60°
(e)   90°

### QUESTION 9**

This and the next two questions pertain to the following situation.

A candle of height 5 cm is placed 4 cm away from a mirror. The image of the candle appears at a distance of 7 cm behind the mirror.

What kind of mirror is being used?

(a)   flat
(b)   convex
(c)   concave

### QUESTION 10*

What is the focal length of the mirror?

(a)   -4 cm
(b)   -5.2 cm
(c)   7.7 cm
(d)   9.3 cm
(e)   11.5 cm

### QUESTION 11*

What is the height of the image?

(a)   2 cm
(b)   3.4 cm
(c)   5.5 cm
(d)   6.9 cm
(e)   8.8 cm

### QUESTION 12**

In empty space, radio waves propagate more slowly than gamma waves.

(T)   True
(F)   False

### QUESTION 13**

A satellite is exposed to sunlight with an rms intensity of 50 kW/m2 at the point in its orbit where it is closest to the sun. What is the rms value of the electric field of the electromagnetic radiation striking the satellite at this point in its orbit?

(a)   1.06 × 10-4 V/m
(b)   0.354 V/m
(c)   120 V/m
(d)   1.45 × 102 V/m
(e)   4.34 × 103 V/m

### QUESTION 14*

This and the next question are conceptually related.

An electromagnetic wave propagating in empty space is found to have a maximum electric field of 40 MV/m. What is the value of the maximum magnetic field to be found with this electric field?

(a)   1.2 × 10-3 T
(b)   0.133 T
(c)   40 T

### QUESTION 15*

This and the next question pertain to the figure shown below.

In which phasor diagram is the instantaneous inductor voltage the greatest?

(a)
(b)
(c)

### QUESTION 16***

Based on the phasor diagrams given, is the generator frequency greater than, equal to or smaller than the resonant frequency of the circuit?

(a)   less than the resonant frequency
(b)   equal to the resonant frequency
(c)   greater than the resonant frequency

### QUESTION 17**

A simple circuit is composed of a resistor (R = 120 Ω), a capacitor (5 μF), an inductor (20 mH), and a generator (Vmax = 12 V, f = 300 Hz) in series.

If the voltage across the inductor is at a maximum, what is the voltage across the capacitor?

(a)   -20.7 V
(b)   -9.2 V
(c)   -6.51 V
(d)   +9.2 V
(e)   +20.7 V

### QUESTION 18*

A man stands in front of a mirror. He has average vision (near point = 27 cm). How close can he stand to the mirror and still see himself in focus?

(a)   13.5 cm
(b)   27 cm
(c)   54 cm

### QUESTION 19*

A ray of light traveling through the air (n = 1) and hits the boundary between air and water (n = 1.33). What is the refracted angle θr if the ray of light is coming in at an angle θi = 30° ?

(a)   0.37°
(b)   0.93°
(c)   22°
(d)   60°
(e)   67°

### QUESTION 20*

A man is standing in front of a mirror getting dressed. His eyes are at a height of 180 cm above the floor. The horizontal distance between him and the mirror is 150 cm. The vertical size of the mirror (top to bottom) is 50 cm.

(a)   0 cm
(b)   50 cm
(c)   75 cm
(d)   90 cm
(e)   150 cm

### QUESTION 21*

If a ray is incident on a h = 1 cm thick sheet of transparent material at an angle θ = 20° is displaced by d = 3 mm as it exits the medium, what is the index of refraction?

(a)   1.33
(b)   1.29
(c)   1.25
(d)   1.19
(e)   1.08

### QUESTION 22**

You are L = 3 m away from a pair of speakers playing a pure tone separated by a distance of d = 1 m. If you walk from the center to a distance of y = 0.75 m, you hear the intensity decrease to zero, then increase all the way back to the initial volume.

What is the frequency of the tone if the speed of sound is 330 m/s?

(a)   2.6 kHz
(b)   1.4 kHz
(c)   1.0 kHz
(d)   800 Hz
(e)   50 Hz

### QUESTION 23*

A resistor in a series RLC circuit dissipates 0.3 W of power when being driven at a frequency of 560 Hz. If R = 100 Ω, L = 175 mH, and C = 0.5 μF, what is the rms voltage of the source?

(a)   7.91 V
(b)   6.06 V
(c)   3.21 V
(d)   1.12 V
(e)   0.761 V

### QUESTION 24*

This and the next two questions pertain to the following situation.

An RLC circuit has R = 110 Ω, XL = 200 Ω and XC = 50 Ω, The circuit is supplied with 120 V (RMS) at 60 Hz.

What is the total impedance of the circuit?

(a)   102 Ω
(b)   186 Ω
(c)   224 Ω
(d)   273 Ω
(e)   305 Ω

### QUESTION 25*

What is the inductance of the inductor?

(a)   0.53 H
(b)   15 H
(c)   75 kH

### QUESTION 26**

Suppose the circuit's frequency were adjusted so that the circuit was in resonance. What would be the maximum instantaneous power dissipated by the circuit?

(a)   90 W
(b)   156 W
(c)   262 W

Scoring of True-False (T,F) Questions:
• If you mark the correct answer, you get 2 points.
• If you mark the wrong answer or if you mark neither answer, you get 0 points.
Scoring of Multiple Choice I (a,b,c) Questions:
• If you mark the correct answer, you get 3 points.
• If you mark a wrong answer or if you mark none of the answers, you get 0 points.
Scoring of Multiple Choice II (a,b,c,d,e) Questions:
• If you mark one answer and it is the correct answer, you get 6 points.
• If you mark two answers and one of them is the correct answer, you get 3 points.
• If you mark three answers and one of them is the correct answer, you get 2 points.
• If you do not mark the correct answer, or if you mark more than three answers, or if you mark none of the answers, you get 0 points.