### Fall 2008 Physics 102 Hour Exam 2(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 109. The exam period was 90 minutes; the mean score was 91.4; the median was 93. 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*

Which one of the following has the shortest wavelength?

(a)   red light
(b)   blue light
(c)   green light

### QUESTION 2*

This and the next question pertain to the the following situation.

Orange light has a wavelength of 6 × 10-7 m in vacuum. What is its frequency? The speed of light is 3 × 108 m/s.

(a)   2 × 1015 Hz
(b)   2 × 10-15 Hz
(c)   5 × 1014 Hz
(d)   5 × 10-14 Hz
(e)   2 × 1014 Hz

### QUESTION 3*

What is the wavelength of orange light in glass (n = 1.5) ?

(a)   4 × 10-7 m
(b)   6 × 10-7 m
(c)   9 × 10-7 m

### QUESTION 4**

How should the transmission axis of your polarized sunglasses be oriented to reduce the glare from light scattered from the road, assuming you are standing upright.

(a)   horizontally
(b)   vertically

### QUESTION 5*

This and the next question pertain to the the following situation.

A beam of light passes from air, through a sheet of plastic, then into air again. The angle the incident beam makes with the normal is θ1, and the angle the beam makes with the normal as it passes through the plastic is θ2. The index of refraction for air is 1 and the index of refraction for plastic is 2.

What is the value of sin θ2 in terms of θ1?

(a)   sin θ2 = (1/8) sin θ1
(b)   sin θ2 = (1/4) sin θ1
(c)   sin θ2 = (1/2) sin θ1
(d)   sin θ2 = 2 sin θ1
(e)   sin θ2 = sin θ1/2

### QUESTION 6**

What is the minimum incident angle θ1 for which the beam will be totally reflected by the plastic? (Note that sin-1 ≡ arcsin.)

(a)   θ1 = sin-1 (1/4)
(b)   θ1 = sin-1 (1/2)
(c)   θ1 = sin-1 2
(d)   θ1 = 0o
(e)   There is no such angle.

### QUESTION 7*

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

A laser emits short pulses of high-intensity electromagnetic waves. The electric field has an rms value of Erms = 2 × 109 N/C.

What is the average intensity S of the laser beam?

(a)   S = 5.4 × 103 W/m2
(b)   S = 2.4 × 109 W/m2
(c)   S = 3.6 × 1014 W/m2
(d)   S = 1.1 × 1016 W/m2
(e)   S = 2.7 × 1032 W/m2

### QUESTION 8*

What is the average power P of a pulse that passes through a surface of area 1.6 × 10-5 m2 perpendicular to the laser beam?

(a)   P = 3.2 × 103 W
(b)   P = 4.4 × 105 W
(c)   P = 1.2 × 107 W
(d)   P = 5.0 × 109 W
(e)   P = 1.7 × 1011 W

### QUESTION 9**

The laser light has vertical polarization. Two polarizers are inserted into the path of the laser beam. Polarizer P1 has its transmission axis parallel to the horizontal axis. Polarizer P2 has its transmission axis at an angle of 45° relative to the horizontal axis. In which configuration is the transmitted intensity the largest?

(a)   beam passes through P1 first, then P2
(b)   beam passes through P2 first, then P1
(c)   The transmitted intensity is the same in both configurations.

### QUESTION 10*

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

A lady bug is at a distance 5 cm from a concave mirror of focal length f = 10 cm.

What is the image distance di formed in the system?

(a)   di =
(b)   di = -10 cm
(c)   di = -5 cm
(d)   di = 10 cm
(e)   di = 20 cm

### QUESTION 11*

The resulting image is

(a)   real.
(b)   virtual.

### QUESTION 12*

The resulting image is

(a)   upright.
(b)   inverted.

### QUESTION 13*

The lady bug is now moved to the left, away from the mirror, until it is at a great distance (say ~1000 m). What is the new image distance di ?

(a)   di =
(b)   di = -10 cm
(c)   di = -5 cm
(d)   di = 5 cm
(e)   di = 10 cm

### QUESTION 14*

This and the next question pertain to the the following situation.

A dentist's mirror is placed 2 cm from a tooth. An enlarged image is located 5.6 cm behind the mirror.

What kind of mirror is being used?

(a)   flat
(b)   convex
(c)   concave

### QUESTION 15*

What is the focal length f of the mirror?

(a)   f = -8.4 cm
(b)   f = -3.1 cm
(c)   f = 3.1 cm
(d)   f = 8.4 cm
(e)   f =

### QUESTION 16**

A woman with normal vision (near point at 26 cm) stands in front of a flat mirror. What is the closest distance to the mirror where she can stand and still see herself in focus?

(a)   13 cm
(b)   26 cm
(c)   52 cm

### QUESTION 17*

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

An object of height 0.05 meters is located 0.6 meters in front of a thin converging lens. The lens forms an image of the object at point A, which is 0.3 meters beyond the lens.

What is the focal length of the lens?

(a)   0.40 m
(b)   0.20 m
(c)   0.15 m

### QUESTION 18*

What is the height of the image formed at position A?

(a)   0.025 m
(b)   0.050 m
(c)   0.075 m

### QUESTION 19*

When the object is moved slightly closer to the lens, the image will

(a)   increase in size and move closer to the lens.
(b)   increase in size and move farther away from the lens.
(c)   decrease in size and move closer to the lens.
(d)   decrease in size and move farther away from the lens.
(e)   stay in the same place.

### QUESTION 20*

If we add a second converging lens with a focal length of 0.6 meters at point B, where will the final image form?

(a)   0.6 m left of the second lens
(b)   1.2 m left of the second lens
(c)   1.2 m right of the second lens
(d)   0.6 m right of the second lens
(e)   0.9 m right of the second lens

### QUESTION 21*

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

With unaided vision your professor can focus only on objects which lie at distances between 3.0 meters and 0.5 meters

Which type of lens is needed to allow him to see objects beyond 3 meters?

(a)   converging
(b)   diverging

### QUESTION 22*

What is the absolute value of the focal length of the lens needed that will allow the professor to see objects clearly at great distance?

(a)   | f |  =  0.5 m
(b)   | f |  =  1.0 m
(c)   | f |  =  3.0 m

### QUESTION 23**

What power lens does the professor need in his glasses to be able to read a book 0.25 meters in front of his glasses? (Assume the distance between his glasses and his eyes is 0.02 m.)

(a)   P = 4.1 Diopters
(b)   P = 1.9 Diopters
(c)   P = 0.50 Diopters
(d)   P = -1.9 Diopters
(e)   P = -0.50 Diopters

### QUESTION 24*

Light passes down through three layers of transparent materials, as shown at right, each with a different index of refraction. At which interface is total internal reflection possible?

(a)   A (from n = 1.3 to n = 1.4)
(b)   B (from n = 1.4 to n = 1.2)

### QUESTION 25*

A film of oil of thickness t is floating on water. The index of refraction is 1.0, 1.45 and 1.33 for air, oil and water, respectively. A person views the film from the air in a direction normal to the surface.

The person sees that there is constructive interference of reflected light of wavelength 630 nm. What is the minimum thickness of the oil film?

(a)   109 nm
(b)   218 nm
(c)   436 nm

### QUESTION 26*

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

Monochromatic light is incident upon two narrow slits separated by a distance d = 2.3 × 10-3 m before striking a screen 4.3 meters away. The distance between the central and first bright fringe is 1.1 × 10-3 m. Calculate the wavelength of the light. (You may approximate sin(θ) ≈ tan(θ) ≈ θ.)

(a)   λ = 588 nm
(b)   λ = 550 nm
(c)   λ = 444 nm

### QUESTION 27**

The entire apparatus is placed in a tank of water (n = 1.33). The spacing Δx on the screen between the first two dark fringes

(a)   increases.
(b)   decreases.
(c)   stays the same.

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.