Summer 2009 Physics 102 Hour Exam 3(24 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 94. The exam period was 75 minutes; the mean score was 67.1; the median was 66. Click here to see page1 page2 page3 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**

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

Unpolarized light with average intensity I = 200 W/m2 is incident upon a pair of polarizers, the first with transmission axis oriented 68° above the x-axis, and the second with transmission axis oriented 24° above the x-axis.

What is the peak value of the magnetic field in the light before it strikes the first polarizer?

(a)   9.2 × 10-7 T
(b)   1.3 × 10-6 T
(c)   1.8 × 10-6 T

QUESTION 2**

What is the average intensity of the light emerging from the second polarizer?

(a)   23 W/m2
(b)   33 W/m2
(c)   52 W/m2
(d)   68 W/m2
(e)   72 W/m2

QUESTION 3*

Put a third polarizer between the first and second ones. What effect would it have on the light emerging from the second polarizer (the one at 24°)? (Assume we leave the first two polarizers at their original angles.)

(a)   The intensity of light emerging from the second polarizer will either be unaffected by the additional polarizer, or reduced because of it.

(b)   The intensity of light emerging from the second polarizer might be reduced, unaffected, or increased because of the presence of the additional polarizer.

QUESTION 4*

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

A real object is held 32 cm in front of a mirror. You will not be told the location of the image it forms, but you are told that the magnification under these circumstances is 0.7 (note that this magnification is positive).

Is the image formed real or virtual?

(a)   real
(b)   virtual

QUESTION 5*

What is the focal length of this mirror?

(a)   -75 cm
(b)   -13 cm
(c)   13 cm
(d)   32 cm
(e)   75 cm

QUESTION 6**

While camping, you experiment with your glasses, and find that by focusing sunlight with them you can start helpful fires and fry harmless bugs. Would your pair of glasses be usable as simple magnifiers? (In other words, could you use them to focus on objects held so close to your eyes that they would otherwise be blurry?)

(a)   Yes, you could use these glasses as magnifiers.
(b)   No, you could not use these glasses as magnifiers.

QUESTION 7**

You can focus on no object farther than 22 cm. You want to focus on objects as far away as 48 cm (but don't care about anything farther than that, for some reason). What weakest correction will suffice? Find the focal length of the contact lenses you need.

(a)   -41 cm
(b)   -15 cm
(c)   15 cm
(d)   41 cm
(e)   48 cm

QUESTION 8*

An object is held 15 from a lens of focal length f1 = -10 cm. A second lens, of focal length f2 = 10 cm, is 20 cm away from the first. (Note that the drawing may not accurately depict the shapes of the lenses.)

In order to project a clear image onto the screen, how far should the screen be from the second lens? Solve for x.
(a)   3.5 cm
(b)   5.8 cm
(c)   7.2 cm
(d)   16.3
(e)   There is no value of x which would produce a clear image on the screen.

QUESTION 9**

Monochromatic light passing through a single slit (width 0.6 mm) produces, on a screen 2.7 meters away, a central diffraction maximum (the wide bright spot bounded by two dark lines) of width 3.8 mm. What wavelength of light is being used here?

(a)   420 nm
(b)   540 nm
(c)   670 nm
(d)   840 nm
(e)   1080 nm

QUESTION 10**

The diagram shows light moving horizontally through water (index of refraction 1.33) and entering a right-triangular prism of unknown index of refraction n. You wish to use the hypotenuse of the prism as a mirror.

What values of n would guarantee that all the horizontal light is reflected back into the glass? Find the greatest range of values that work.

(a)   n > 1.33
(b)   n > 1.41
(c)   n > 1.88
(d)   n < 1.41
(e)   No value of n would guarantee this.

QUESTION 11*

Light moving through air enters a glass block (index of refraction 1.38), making an initial angle of 33° above the surface, as shown. Find the corresponding angle θ for the refracted light inside the block.

(a)   23.2°
(b)   37.4°
(c)   52.6°

QUESTION 12**

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

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 13**

That same oil is now poured out onto a glass window. Notice that the glass window, being held up on either end, is very slightly sagging in the middle, though it does not break. Take the index of refraction of the glass to be 1.58 .

Do we find a bright spot or a dark spot at the edge of the oil (where the thickness of the oil layer approaches zero)?

(a)   bright
(b)   dark

QUESTION 14**

The thickness of the oil coating varies because of the shape of the glass. At some thickness t the orange light (590 nm) is not reflected. At what next largest thickness is the light again not reflected?

(a)   t + 97 nm
(b)   t + 194 nm
(c)   t + 295 nm

QUESTION 15*

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

A diffraction grating has 3000 lines (slits) per cm.

When illuminated by green light of wavelength 540 nm, at which of these angles is the second-order maximum detected?

(a)   9.3°
(b)   12.4°
(c)   15.6°
(d)   18.9°
(e)   22.2°

QUESTION 16**

True or false: the fourth-order maximum for green light (540 nm) overlaps with (is at the same angle as) one of the maxima for violet light (432 nm).

(T)   True, a violet bright spot coincides with the fourth-order green maximum.
(F)   False, no violet bright spot overlaps the fourth-order green maximum.

QUESTION 17*

The red taillights (wavelength λ = 670 nm) are 1.8 meters apart on a car speeding away from you in the dark. Assuming the pupils of your eyes each have a diameter of 6 mm, how far away can the car drive before the two taillights appear to overlap into a single light? Assume that this maximum distance is determined by diffraction, and not by the density of photoreceptors on your retina.

(a)   130 m
(b)   1300 m
(c)   13 km

QUESTION 18*

Light of wavelength 600 nm is incident on a pair of slits separated by distance 0.1 mm. What is the largest order of interference maximum we would expect to observe?

(a)   33
(b)   66
(c)   166

QUESTION 19**

A single slit of width 0.5 mm is illuminated by monochromatic light. By inserting a barrier of width just under 0.5 mm, we can create a pair of very narrow slits with separation 0.5 mm. The barrier blocks all light except for the light passing just above or below it.

Compare what happens with the barrier (creating two slits) versus without the barrier (a single wide slit).

(a)   Without the barrier, there is a bright spot at θ = 0, which disappears when the barrier is inserted.

(b)   With the barrier, there is a bright spot at θ = 0, which disappears when the barrier is removed.

(c)   There is always a bright spot at θ = 0, regardless of whether the barrier is present or not.

QUESTION 20**

Two lenses (the first with focal length 36 cm, the second with focal length 18 cm) are separated by 80 cm. An object is held 90 cm in front of the first lens. Which one of these descriptions accurately compares the final image to the initial object?

(a)   The final image is real, enlarged, and upright.
(b)   The final image is virtual, enlarged, and inverted.
(c)   The final image is real, enlarged, and inverted.
(d)   The final image is virtual, reduced, and inverted.
(e)   The final image is real, reduced, and inverted.

QUESTION 21**

On the hard, smooth surface of a tiled floor, you can see the reflected glare from a light bulb located at the far end of a large room. By holding a polarizer in front of your face, you make that glare disappear. How is the polarizer oriented?

(a)   The transmission axis of the polarizer is horizontal.
(b)   The transmission axis of the polarizer is vertical.

QUESTION 22*

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

This diagram shows two principal rays after having been reflected by a concave mirror. (The principal ray which passes through the center of curvature is not shown.) The two upright arrows are not part of the diagram but they are drawn to the same scale as the diagram. You should assume that these rays of light started, before reflecting, at the top of a single real object; the bottom of that object rests on the principal axis (the dashed line).

One of those two arrows has the same height as the object in this problem. Which one?

(a)   A
(b)   B

QUESTION 23*

Is the focal length of this mirror positive or negative?

(a)   positive
(b)   negative

QUESTION 24**

Which one of these descriptions is completely true?

(a)   The image is virtual because the object is to the left of the focal point.
(b)   The image is real because the object is to the left of the focal point.
(c)   The object is at the focal point.
(d)   The image is virtual because the object is to the right of the focal point.
(e)   The image is real because the object is to the right of the focal point.

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.