Fall 2006 Physics 102 Hour Exam 3
(27 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 115. The exam period was 90 minutes; the mean was 85.3; the median was 88. Click here to see page1 page2 of the formula sheet that came with the exam.


QUESTION 1**

A transmitting antenna oriented parallel to the ground and pointing in a north-south direction broadcasts radio waves, as shown in the figure. How should one orient the antenna of a radio in order to receive the strongest signal?

(a)   parallel to the ground along an east-west direction
(b)   parallel to the ground along a north-south direction
(c)   normal to the ground


QUESTION 2**

What should the minimum area of a satelliteís solar panels be in order to provide 20 kW of electrical power? Assume the solar panels convert the energy of sunlight into usable electrical power with 23% efficiency. The rms value of the electric field in sunlight hitting the satellite is 720 N/C.

(a)   9.4 m2
(b)   17.0 m2
(c)   38.3 m2
(d)   63.2 m2
(e)   173.7 m2


QUESTION 3**

This and the next question concern the same situation.

A car is approaching an intersection with traffic lights at a constant speed. An observer at rest relative to the traffic lights sees a yellow light. At what speed would the carís driver see a green light? Assume the wavelengths of yellow and green light are 570 and 550 nm, respectively.

(a)   650 km/hour
(b)   750 km/sec
(c)   10909 km/sec


QUESTION 4***

If the car continued to move toward the traffic lights but changed speed, at what speed would the yellow light be seen by the driver as a red light with wavelength 625 nm?

(a)   at a speed higher than above
(b)   at a speed smaller than above
(c)   The driver would not see a red light at any speed.


QUESTION 5*

This and the next question concern the same situation.

Unpolarized light passes through two polarizers oriented as shown in the figure. The intensity of light coming out from the second polarizer S2 is 33% of the intensity of the unpolarized light S0.

What is the angle between the transmission axes of the polarizers?

(a)   7.0°
(b)   24.9°
(c)   35.7°
(d)   43.5°
(e)   79.0°


QUESTION 6**

Can one increase the intensity of light transmitted by S2 by rotating the first polarizer around the direction of the light?

(a)   Yes
(b)   No


QUESTION 7*

An object P is placed in front of a convex mirror as show in the figure. Which of A, B, C, D or E best represents its image? The focal point is indicated as F.

(a)   
(b)   
(c)   
(d)   
(e)   


QUESTION 8*

This and the next question concern the same situation.

An object P is placed 3 meters away from a concave mirror as shown in the figure. The focal length of the mirror is 5 meters.

What is the magnitude of the distance from the mirror to the image?

(a)   2.0 m
(b)   3.0 m
(c)   5.2 m
(d)   5.0 m
(e)   7.5 m


QUESTION 9*

The image is

(a)   real and inverted.
(b)   virtual and inverted.
(c)   virtual and upright.


QUESTION 10***

A ray of light is traveling toward two flat mirrors arranged at a 30° angle, as shown in the figure. The incident light is traveling parallel to the upper mirror. How many reflections will the ray make before escaping?
(a)   3
(b)   5
(c)   The ray will never escape from the corner.


QUESTION 11*

A convex mirror has a radius of curvature of 10 meters. What is the focal length of the mirror?

(a)   10 m
(b)   5 m
(c)   -5 m


QUESTION 12*

This and the next question concern the following situation:

An electromagnetic wave is traveling in the direction of x axis. The electric field of the wave oscillates within the xy plane. The amplitude of the electric field is 45 V/m.

What is the amplitude B0 of the magnetic field of the wave?

(a)   3.2 T
(b)   0.02 mT
(c)   1.5 × 10-7 T


QUESTION 13***

A circular loop of wire is introduced into the wave. Which orientation of the loop will have the largest induced emf?

(a)   parallel to the xz plane
(b)   parallel to the yz plane
(c)   parallel to the xy plane


QUESTION 14*

A convex lens with focal length f = 10 cm is used to produce an inverted, real image of a candle that has been placed to the left of the lens. The image is found to be three times the size of the candle used as the object.

What is the distance do from the object to the lens?

(a)   do = 0.33 cm
(b)   do = 10.00 cm
(c)   do = 13.33 cm
(d)   do = 30.00 cm
(e)   do = 47.21 cm


QUESTION 15**

In the following diagram, a convex lens with focal length f1 and concave lens with focal length f2 are aligned along as shown. A candle is placed to the left of the convex lens. The image formed by the convex lens serves as the object for the concave lens. By drawing appropriate rays as necessary, determine which of the indicated positions a, b, c, d, or e is closest to the location of the image produced by the concave lens.

(a)   
(b)   
(c)   
(d)   
(e)   


QUESTION 16*

This and the next question concern the following situation:

The eyes of a myopic (nearsighted) patient in need of corrective lenses are somewhat prolate so that the distance from the cornea to the retina is greater than the focal length of the combination of the cornea and relaxed lens, as shown in the figure. As a result, parallel rays from a very distant object will focus 25 mm from the front of the eye, somewhat in front of the retina.

A myopic physicist can focus comfortably on objects that are 150 mm from the front of his eye when the lens of his eye is relaxed. What is the distance from the front of the physicistís eye to his retina? (Assume the focal length of the combination of the cornea and relaxed lens is 25 mm.)

(a)   30 mm
(b)   27 mm
(c)   31 mm


QUESTION 17*

Recall that the power of a lens is sometimes expressed in diopters, where the diopter value is the same as the inverse of the lensí focal length in meters: diopters = 1/f. Imagine that a myopic patient can see objects clearly without wearing corrective lenses as long as the objects are no farther than 200 mm away. The prescription for corrective lenses worn by this patient specifies lenses of

(a)   +1.00 diopter
(b)   +2.50 diopters
(c)   -0.20 diopters
(d)   -3.20 diopters
(e)   -5.00 diopters


QUESTION 18**

The magnifying glass carried by Sherlock Holmes is an example of a simple magnifier. (You might use a magnifying glass to inspect a used CD for scratches at a music store.) The simple magnifier works by

(a)   producing a real image that is closer to the eye than the real object being inspected with the aid of the magnifier.
(b)   producing a virtual image that is farther from the eye than the real object being inspected with the aid of the magnifier.
(c)   reinverting the otherwise once-inverted image formed by the lens in order to make inspection possible.


QUESTION 19**

As shown in the figure, light of wavelength λ reflects off the front and back faces of a very thin sheet of glass of thickness d. The glass is so much thinner than the wavelength of the incident light that the distance the light travels inside the glass is insignificant compared to λ.

(a)   The interference between the two rays of reflected light drawn in the figure is constructive.
(b)   The interference between the two rays of reflected light drawn in the figure is destructive.


QUESTION 20*

Two sheets of transparent material with indices of refraction n2 and n3 are stacked as shown in the figure. The bottom face of the lower sheet is coated with a light-absorbing material so that no light can reflect off this face.

Laser light of wavelength λ = 550 nm in air strikes the top surface of the upper sheet. The distance the light travels inside the sheet with index of refraction n2 is 2d. Note that n3 > n2. When the laser beam is aimed as shown in the figure, the reflected light is seen to be especially bright. Which value of d can produce a bright reflected beam?

(a)   d = 250 nm
(b)   d = 375 nm
(c)   d = 500 nm
(d)   d = 183 nm
(e)   d = 83 nm


QUESTION 21***

Two narrow slits spaced by 10 mm in an opaque barrier are illuminated by a pair of very bright light bulbs, as shown in the figure. A viewing screen is located 200 m to the right of the barrier. There are no other sources of light that can illuminate the screen.

Which one of these intensity patterns best illustrates the illumination pattern that might be seen on the screen? Note the tick mark that indicates the center of the screen.

(a)   
(b)   
(c)   


QUESTION 22***

Two narrow slits are spaced 1 m apart in a solid, opaque barrier as shown in the following figure. Behind one slit is a microwave generator beaming microwaves with wavelength λ = 10 cm at the slit. Behind the other slit is a loudspeaker emitting a pure sinusoidal tone, also with wavelength λ = 10 cm. An audio microphone is placed some distance away from the two slits so that one slit is 5 cm closer to the microphone than the other. At t = 0 the microwave generator is turned off so that only the loudspeaker is operating.

When the microwave generator is turned off, how does the intensity of sound registered by the audio microphone change?

(a)   Sound at the microphone is louder.
(b)   Sound at the microphone is unchanged.
(c)   Sound at the microphone is softer.


QUESTION 23*

Two narrow slits spaced by 0.1 mm in an opaque barrier are illuminated by an electromagnetic wave from a laser, as shown in the figure. A viewing screen is located 5 m to the right of the barrier. The first minimum in the interference pattern is 10 mm from the center of the central bright spot. Note that the various features in the figure are not drawn to scale.

What is the wavelength λ of the laser light?

(a)   λ = 400 nm
(b)   λ = 350 nm
(c)   λ = 800 nm
(d)   λ = 625 nm
(e)   λ = 550 nm


QUESTION 24*

A ray of light hits a series of three interfaces as shown below. At what value of θ1 will it undergo total internal reflection at the third (last) interface?

(a)   1.25°
(b)   21.06°
(c)   44.22°
(d)   70.63°
(e)   86.43°


QUESTION 25**

A ray of unpolarized light traveling in air reflects off an interface with index of refraction n = 1.33. At what angle θ is the reflected beam perfectly polarized?

(a)   18°
(b)   42°
(c)   53°


QUESTION 26*

This and the next question concern the following situation:

An object with height h = 8 cm lies a distance d = 32 cm before a diverging lens with focal length f = -25 cm.

What is the size of the image?

(a)   2.22 cm
(b)   3.51 cm
(c)   8.46 cm
(d)   14.2 cm
(e)   25.5 cm


QUESTION 27*

Which one of the following is true?

(a)   The image is real and on the left side of the lens.
(b)   The image is inverted and virtual.
(c)   The image is upright and reduced.
(d)   The image is virtual and on the right side of the lens.
(e)   The image is real and reduced.