### Spring 2002 Physics 102 Hour Exam 3(29 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 ***.

This exam consists of 29 questions; 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 91. The exam period was 90 minutes; the mean was 75.9; the median was 78. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1*

This and the next question are about the following situation:

A person is nearsighted (myopic) and without glasses the furthest distance that she can see an object clearly is 66.4 cm.

You would prescribe contact lenses with

(a)   negative focal length.
(b)   positive focal length.

### QUESTION 2*

The power of the contact lenses should be:

(a)   |P| = 1.47 diopters
(b)   |P| = 1.51 diopters
(c)   |P| = 1.55 diopters

### QUESTION 3*

A convex mirror produces a real image of a real object

(a)   never.
(b)   only when the object is between the focal point and the mirror.
(c)   always.

### QUESTION 4**

A person looks into the water at a fish she knows is located 3 meters directly below her. The fish appears to be 20 cm long. How big is the fish in reality? (As shown, her eyes are close to, but above the surface of the water; n=1.33).

(a)   15 cm
(b)   20 cm
(c)   25 cm

### QUESTION 5*

This and the next question are about the following situation:

A light beam enters a plastic slab (n = 1.6, d = 10 cm) with an incident angle 30o, at point A and exits at point B (see figure).

Calculate the horizontal distance x the beam travels in going from A to B?

(a)   x = 3.3 cm
(b)   x = 4.4 cm
(c)   x = 5.7 cm

### QUESTION 6*

Calculate the time t it take for the beam to cover the distance AB?

(a)   t = 0.33 ns
(b)   t = 0.44 ns
(c)   t = 0.56 ns

### QUESTION 7**

The figure shows a prism made out of a material with unknown index of refraction. What is the minimum index of refraction this material can have such that a horizontal beam, incident from the left, undergoes total internal reflection as shown in the figure?

(a)   1.33
(b)   1.56
(c)   1.83

### QUESTION 8**

This and the next three questions concern the following situation:

An object is located between the focal point f and twice f at the left of a converging lens made from glass (n = 1.5), as shown in the figure.

The image is located

(a)   between the lens and C.
(b)   between C and D.
(c)   to the right of D.

The image is

(a)   reduced.
(b)   enlarged.

The image is

(a)   inverted.
(b)   upright.

### QUESTION 11*

The glass lens is now replaced by a diamond lens (n = 2.4) with exactly the same shape. The image location will now be

(a)   closer to the lens.
(b)   farther from the lens.
(c)   in the same position as before.

### QUESTION 12*

This and the next four questions concern the following situation:

In the figure is shown a two-lens system. Lens #1 is a converging lens with f1 = 15 cm, while Lens #2 is a diverging lens with f1 = -10 cm. They are placed 7 cm apart. An object of height 5 cm is placed 40 cm to the left of lens #1.

The image produced by ONLY the Lens #1 is a distance di1 (from O1).

(a)   di1 = -24 cm (to the left of lens 1)
(b)   di1 = -18 cm (to the left of lens 1)
(c)   di1 = -12 cm (to the left of lens 1)
(d)   di1 = +12 cm (to the right of lens 1)
(e)   di1 = +24 cm (to the right of lens 1)

### QUESTION 13*

The image produced by ONLY the Lens #1 has a height hi1

(a)   |hi1| = 3 cm
(b)   |hi1| = 5 cm
(c)   |hi1| = 8 cm

### QUESTION 14*

The image produced by both lenses is a distance di2 (from O2).

(a)   di2 = –24.3 cm (to the left of lens 2)
(b)   di2 = –20.1 cm (to the left of lens 2)
(c)   di2 = –17.8 cm (to the left of lens 2)
(d)   di2 = +17.8 cm (to the right of lens 2)
(e)   di2 = +20.1 cm (to the right of lens 2)

### QUESTION 15*

The image produced by both lenses has a height hi2

(a)   |hi2| = 1.8 cm
(b)   |hi2| = 2.7 cm
(c)   |hi2| = 4.3 cm

### QUESTION 16**

The image produced by both lenses is

(a)   upright.
(b)   inverted.

### QUESTION 17*

This and the next two questions concern the following situation:

Monochromatic light hits a barrier with two slits separated by distance d = 0.2 mm. The two slits act as two point-like sources of light. When the light hits a screen a distance L=4m away, the second dark fringe is y=1.6 cm from the center of the screen. Assume that the two slits are sufficiently narrow so that you can ignore their widths.

What is the wavelength of the light?

(a)   488 nm
(b)   514 nm
(c)   532 nm

### QUESTION 18*

If the surrounding medium is filled with water (n = 1.33), what will be the new distance between the second dark fringe and the center of the screen?

(a)   2.1 cm
(b)   1.6 cm
(c)   1.2 cm

### QUESTION 19*

If the slits are brought closer to each other, in which direction will the second dark fringe above the center move? (See diagram above.)

(a)   Up.
(b)   Down.
(c)   It does not move.

### QUESTION 20*

This and the next three questions concern the following situation:

A thin (640 nm) layer of water (n = 1.33) rests on a glass surface (n = 1.5). Light with wavelength (in air) of 568 nm is incident from above.

Light reflecting off of the top and bottom layers of the water will interfere

(a)   constructively.
(b)   destructively.

### QUESTION 21**

As the water evaporates, the thickness will continue to decrease. How many destructive interferences will occur as the water thickness decreases from 640 nm to 10 nm?

(a)   2
(b)   3
(c)   4

### QUESTION 22*

What is the minimum, non-zero thickness of the water layer that will produce the constructive interference? (Be careful all of the answers below can be obtained from numbers given in the problem.)

(a)   95 nm
(b)   107 nm
(c)   189 nm
(d)   213 nm
(e)   320 nm

### QUESTION 23**

Now imagine that the glass surface is replaced by air. That is, the water layer now forms a free-standing film 640 nm thick. Which of the following statements is correct?

(a)   The light of 568 nm will be constructively interfered upon reflections of the film.
(b)   The light of 568 nm will be destructively interfered upon reflections of the film.
(c)   The interference of 568 nm upon reflections will be neither constructive nor destructive.

### QUESTION 24*

This and the next question concern the following situation:

Light of 488 nm wavelength is incident on a grating made with 8000 lines/cm. The distance between the grating and the screen is 25 cm.

What distance above the center of the screen will the first bright fringe occur?

(a)   4.8 cm
(b)   7.2 cm
(c)   10.6 cm

### QUESTION 25**

What is the maximum order of bright fringes that can be formed by this grating?

(a)   2
(b)   3
(c)   4

### QUESTION 26*

A camera of 0.2 cm aperture diameter is used to take a picture of a car driving toward the camera. If the headlights of the approaching car are separated by 1.5 m, what is the maximum distance between the car and the camera at which the picture taken by the camera can fully resolve the two headlights? For this problem, assume that the headlights have monochromatic source (wavelength = 580 nm).

(a)   4.2 km
(b)   5.1 km
(c)   5.8 km

### QUESTION 27*

This and the next two questions concern the following situation:

A person needs a bifocal lens in order to read books at 30 cm distance away from the eyes and also to see objects far away. The near point of the person’s eye is 0.5m. (Typically, a bifocal lens allows people to read books using the bottom part of the lens.)

Which one of the following pictures best represent a bifocal lens?

(a)
(b)
(c)

### QUESTION 28*

If the bifocal lens is 1.5 cm away from the eye, what is the absolute value of the focal length of the bottom lens?

(a)   69 cm
(b)   99 cm
(c)   108 cm

### QUESTION 29*

If the absolute value of the focal length of the top lens is 100 cm, what is the approximate value of the far point distance?

(a)   75 cm
(b)   100 cm
(c)   125 cm

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.