### Spring 2001 Physics 102 Hour Exam 3(28 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 28 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 107. When the exam was given, the mean was 83.8; the median was 86. Click here to see the formula sheet that came with the exam.

### QUESTION 1*

When you look at your reflection in a flat mirror, the image you see is virtual.

(T)   True
(F)   False

### QUESTION 2*

An object is placed in front of a spherical mirror. The image produced is virtual and smaller. Is the mirror concave or convex? (hint, draw some examples)

(a)   Concave.
(b)   Convex.
(c)   Both concave and convex could produce this image.

### QUESTION 3*

An unknown light source shines through a diffraction grating and produces a first-order bright fringe at a height h = 0.894 m above the central bright fringe on a flat screen located 0.625 meters away.

What is the wavelength l of the incident light?

(a)   l = 591 nm
(b)   l = 654 nm
(c)   l = 712 nm
(d)   l = 759 nm
(e)   l = 808 nm

### QUESTION 4*

This and the next two questions are about the following situation:

An object is placed between the center of curvature, C, and the focal point, F, of a concave mirror. Use ray-tracing techniques to locate the image.

The image will be found closest to:

(a)   I
(b)   II
(c)   III
(d)   IV
(e)   V

The image is:

(a)   real
(b)   virtual

### QUESTION 6**

If the distance from the tail of the arrow to the optical axis is 1 cm, the distance from the tail of the image arrow to the optical axis will be:

(a)   less than 1 cm
(b)   also equal to 1 cm
(c)   greater than 1 cm

### QUESTION 7*

This and the next question are about the following situation:

An object is placed in front of a convex mirror as shown. (Warning: incorrect answers can be obtained from the numbers in the problem.)

Where will the image be located with respect to the mirror?

(a)   di = -24 cm
(b)   di = +24 cm
(c)   di = +120 cm

### QUESTION 8*

What is the magnification, M, of the mirror for the object placed as shown?

(a)   M = -2.0
(b)   M = -0.4
(c)   M = +0.4
(d)   M = +1.0
(e)   M = +2.0

### QUESTION 9*

This and the next question are about the following situation:

Which ray in the ray diagram above is not correct?

(a)   1
(b)   2
(c)   3

### QUESTION 10*

Where would the image be formed in the picture above?

(a)   to the left of the left focal point
(b)   between the left focal point and the lens
(c)   between the right focal point and the lens
(d)   at the right focal point
(e)   to the right of the right focal point

### QUESTION 11*

This and the next question are about the following situation:

A converging lens and a diverging lens are placed as shown in the figure above, 12 cm apart. The converging lens is located at the origin. An object is located at –50 cm.

Where along the principle axis is the location of the image formed by only the first lens?

(a)   -38.2 cm
(b)   –12.5 cm
(c)   +33.3 cm

### QUESTION 12**

Where is the location of the final image due to both lenses. Give your answer along the principle axis shown above with the origin at the converging lens.

(a)   -33.3 cm
(b)   -17.0 cm
(c)   -6.8 cm
(d)   +5.6 cm
(e)   +29.7 cm

### QUESTION 13*

A light beam travels from material 1 to 2 to 3 along the path shown in the figure to the right. From this picture, you can conclude that:

(a)   n1 < n2 < n3
(b)   n1 < n3 < n2
(c)   n2 < n3 < n1

### QUESTION 14*

This and the next question are about the following situation:

In the figure to the right, a light beam starts from the lower left corner, in air, enters a box with a transparent material inside, strikes a mirror and reflects, then exits the box back into the air along the direction shown.

If the index of refraction of the material in the box is increased from nstuff = 1.4 to nstuff = 1.6, the distance L will

(a)   increase.
(b)   decrease.
(c)   remain the same.

### QUESTION 15*

For nstuff = 1.5, calculate L.

(a)   L = 6.39 cm
(b)   L = 7.14 cm
(c)   L = 8.05 cm
(d)   L = 8.88 cm
(e)   L = 9.32 cm

### QUESTION 16*

This and the next two questions are about the following situation:

A man requires bifocals to correct both his near and far vision problems. Consider the bifocals to be a pair of glasses with a "main" lens and an additional "add-on" lens placed at the bottom. The main lens corrects his vision of distant objects. The add-on lens corrects his vision for reading.

Which of the following figures qualitatively represents the lenses in a pair of bifocals as described above?

(a)
(b)
(c)

### QUESTION 17*

The absolute value of the power of the main lens is | P | = 2.4 diopters. Without the glasses, how far away can the man see clearly?

(a)   41.6 cm
(b)   53.9 cm
(c)   66.3 cm

### QUESTION 18*

Looking through only the main lens, our man can read a book no closer than 35 cm away. But, he would like to read at a distance of 25 cm. What is the absolute value of the power of the add-on lens which will permit him to do this?

(a)   | P | = 1.05 diopters
(b)   | P | = 1.09 diopters
(c)   | P | = 1.14 diopters
(d)   | P | = 1.23 diopters
(e)   | P | = 1.40 diopters

### QUESTION 19**

This and the next three questions pertain to the following information:

A light source inside an optical fiber emits light rays with equal intensity in all directions. The fiber has a core surrounded by a cladding. The combination of core/cladding materials that has the greatest transmission of light is:

(a)   Plexiglass / Crown Glass
(b)   Polystyrene / Fused quartz
(c)   Dense flint glass / Light Flint Glass

### QUESTION 20*

How far does yellow light (l = 580 nm) travel in water in 2 seconds?

(a)   4.5 × 108 m
(b)   6.0 × 108 m
(c)   8.0 × 108 m

### QUESTION 21*

Using a block of a transparent, unknown material, it is found that a beam of light inside the material is totally internally reflected at the air-block interface at an angle of 41.5° . What is the material?

(a)   Water
(b)   Fused Quartz
(c)   Plexiglass

### QUESTION 22**

A converging lens made of fused quartz has a focal length of 10 cm. A second lens, made of light flint glass, has the exact same physical shape and size as the quartz lens. Its focal length will be

(a)   < 10 cm.
(b)   = 10 cm.
(c)   > 10 cm.

### QUESTION 23*

This and the next two questions are about the following situation:

Red light (l = 600 nm in air) impinges at normal incidence on a thin plastic film (n=1.4) which is originally on top of a thick glass plate (n=1.5).

Find t, the minimum (non-zero) thickness of the film which gives constructive interference. (Warning: incorrect answers can be obtained from the numbers in the problem.)

(a)   t = 100 nm
(b)   t = 107 nm
(c)   t = 200 nm
(d)   t = 214 nm
(e)   t = 300 nm

### QUESTION 24*

If the thickness of the plastic film were doubled, the red light would interfere

(a)   constructively.
(b)   destructively.

### QUESTION 25*

If the entire apparatus were placed in a tank of water (n=1.33), your answer to the previous question would

(a)   remain the same.
(b)   change to the opposite choice.

### QUESTION 26***

This and the next two questions are about the following situation:

Blue light with wavelength l = 380 × 10-9 m is incident upon two narrow slits separated by a distance d before striking a screen 5.2 meters away. The distance between the central bright fringe and first dark fringe is 1.8 × 10-3 m. Calculate d the spacing between the slits. You may approximate sin(q) » tan(q) » q .

(a)   d = 0.55 × 10-3 m
(b)   d = 1.10 × 10-3 m
(c)   d = 1.65 × 10-3 m
(d)   d = 2.20 × 10-3 m
(e)   d = 4.40 × 10-3 m

### QUESTION 27*

Now red light (l = 650 × 10-9) is incident on the same two slits, the distance y between the central bright fringe and the first dark fringe is

(a)   y < 1.8 mm
(b)   y = 1.8 mm
(c)   y > 1.8 mm

### QUESTION 28***

Now the two slits are replaced by a single slit with width w = d, and illuminated with the original blue light. What is the distance y to the first dark fringe?

(a)   y < 1.8 mm
(b)   y = 1.8 mm
(c)   y > 1.8 mm

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.