Answers to multiple choice questions are given at the end.  Here is an equation sheet.

											

PHYS 352 Spring 1999
	Exam 2		mult. choice:_______
 									prob 1:      ________
									prob 2:      ________

									TOTAL:     ________
Name ____________________					March 30, 1999
	Please Print

You are expected to observe the GMU honor code.  Please read these instructions carefully:
1. You may be pressed for time, so don't waste time on things you get stuck on.
2. There's no penalty for guessing on the multiple choice part, so don't leave any blank.
3. Multiple choice questions count 4 points each, for a total of 60 points for that part.
4. Be sure to read each question VERY carefully.  Experience shows that a major reason people answer questions wrong is that they don't read them correctly.

1. When light passes from air into glass (n = 1.5) what happens to its frequency, wavelength and velocity?
(a) all decrease by a factor 1.5,      (b) f and v decrease by a factor 1.5
(c) f and ë decrease by a factor 1.5,  (d) ë and v decrease by a factor 1.5
(e) none of the above

2. Due to total internal reflection an underwater swimmer looking up at the surface of the water from a depth of 10 ft sees a mirrored surface with a hole in it of radius R.  Find R if the index of refraction of water is 1.33.
(a) 5.67 ft,  (b) 7.46 ft,  (c) 9.12 ft,  (d) 11.3 ft,  (e) none of them

3. What type of mirror produces a real image of magnification M = - 4 when an orbject is located 10 cm in front of it?
(a) a convex mirror of radius 4 cm,   (b) a concave mirror of radius 4 cm
(c) a convex mirror of radius 16 cm,  (d) a concave mirror of radius 16 cm
(e) none of the above

4. Two lenses A and B have identical shapes, but the index of refraction of A is 1.5 and that of B is 1.6.  What is the ratio of the focal length of B to that of A?
(a) 1.2,  (b) 0.833,  (c) 1.067,  (d) 0.938,  (e) none of the preceding

5. Which of the following only produce virtual images?  (Assuming s > 0)
(a) converging lenses and concave mirrors, (b) diverging lenses and concave mirrors
(c) converging lenses and convex mirrors, (d) diverging lenses and convex mirrors
(e) none of the above

6. In the Lloyd's mirror experiment the light source and its mirror image are:
(a) coherent and in phase,  (b) coherent and out of phase,   
(c) incoherent and in phase,  (d) incoherent and out of phase,  (e) none of them

7. Counting only angles greater than or equal to zero and less than or equal to 90 degrees, how many maxima will be found when microwaves of wavelength 2 cm are incident on a pair of slits whose separation is 9 cm?
(a) 3,  (b) 4,  (c) 5,  (d) 6,  (e) none of them   

8. What is the spacing between adjacent bright interference fringes when a pair of slits 0.1 mm apart are illuminated by light of wavelength 500 nm, and the fringes are observed on a screen 2 m behind the slits?
(a) 0.5 cm,  (b) 1.0 cm,  (c) 2.0 cm,  (d) 10.0 cm,  (e) none of them

9. When you look at a soap film on a vertical wire hoop you see horizontal interference fringes in different colors.  Over time the soap film thickness decreases at the top and increases at the bottom.  This will cause
(a) the spacing between the fringes to increase, (b) the spacing between the fringes to decrease
(c) the fringes to cease being horizontal, (d) the fringes to become less colored
(e) none of the above

10. Which of the following will exhibit the greatest amount of diffraction?
(a) light waves incident on a human hair, (b) light waves incident on a 1 cm hole,
(c) sound waves incident on a 1 cm hole, (d) sound waves incident on a doorway,
(e) none of the above

11. How close together in angle could a pair of stars be, and still be resolved if their dominant wavelength is 500 nm, and they are viewed through a telescope whose objective lens has a diameter of 50 cm?
(a) 1.22 x 10-6 rad,  (b) 1.22 x 10-4 rad, (c) 10-6 rad,  (d) 10-4 rad, (e) none of them

12. How many slits would a diffraction grating need to be able to resolve two wavelengths: 500.0 nm and 500.1 nm in the second order?
(a) 5000,  (b) 50,000,   (c) 2500,   (d) 25,000,   (e) none of them

13. When you look at a light source through a polaroid filter and rotate the filter one full revolution you observe that the light intensity changes, but it never drops to zero.  From this observation you can conclude that the light from the source was
(a) unpolarized,  (b) completely polarized, (c) partly polarized, (d) coherent

14. Find the focal length of a lens that forms an inverted image the same size as the object when the object is located 20 cm in front of the lens.
(a) 10 cm,  (b) 30 cm,  (c) 40 cm,  (d) 50 cm,  (e) none of them

15. A light ray originating inside a transparent liquid is incident at an angle of 45 degrees and is totally internally reflected when it strikes the surface of the liquid.  From this we can conclude that the index of refracrtion of the liquid is:
(a) equal to 1.414,  (b) greater than 1.414,  (c) less than 1.414,  (d) none of them

Problem 1

10 pts (a) Astronomers receive a 60 MHz radio source both directly and by reflection from the sea.  If the receiving dish is 20 m above sea level, what is the angle of the radio source above the horizon for the first order maximum?  (Hint: Is there a phase change here?)

10 pts (b) A possible means of making an airplane invisible to radar is to coat the plane with an antireflecting polymer.  If radar waves have a wavelength of 3 cm and the index of refraction of the polymer is 1.5, how thick would you make the coating?  (Let n for plane exceed 1.5)

10 pts (c) A hair is placed at one edge between two flat glass plates 8 cm long.  When this arrangement is illuminated from above with 600 nm light, 121 dark bands are counted, starting at the point of contact of the two plates.  How thick is the hair?

Problem 2.

Neatness counts for all three parts of this problem!

10 pts (a) Draw a ray diagram showing the 3 principal rays and the location of the image for an object located 5 cm in front of a converging lens of 10 cm focal length.

10 pts (b) Explain with the aid of two diagrams why light rays entering one end of an optical fiber get trapped inside and emerge at the other end, and also why the fiber must not be bent through too large an angle.

10 pts (c) Draw a diagram showing the light rays that reach your two eyes when you look down at a fish underwater, and explain with the aid of the diagram why the fish appears to be at a shallower depth than it actually is.

Answers to multiple choice:
1 d,  2 d,  3 d,  4 b,  5 d,  6 b,  7 c,  8 b,  9 b,  10 c,  11 a,  12 c,  13 c,  14 a,  15 b