Physics 112 – Test #6
17-18 December 2013
Name_____________________
Directions: This is a CLOSED BOOK test, and you may only use the calculator provided.
SHOW ALL OF YOUR REASONING! You can only get partial credit for an incorrect
answer if you show your reasoning. You may have as much time as you like to finish this
exam.
1. (7 points) Most eyeglasses nowadays have an anti-glare coating on them, including
your prof’s. When he looks almost straight down on them with a white light source
directly overhead, he sees a reflection of the light source but it appears green (λ = 520
nm). When he looks at the reflection of a white light source that is off to the side, it
appears red (λ = 650 nm). (If you would like to see this for yourself, you may.)
white light source
white light source
RAT sees green!
Why does the reflection look green in one case but red in the other?
RAT sees red!
2. Unpolarized light of intensity 50 W/m2 (that’s bright!) is incident on some ideal
polarizers.
In situation A, the unpolarized light hits a polarizer with its polarization axis oriented
vertically. It goes through this polarizer and is then incident on a polarizer with its
polarization axis oriented horizontally.
In situation B, the unpolarized light goes through a polarizer with its polarization axis
oriented vertically. It then goes through one with its axis oriented 40o with respect
to vertical, and then through another one with its axis oriented horizontally.
A
B
40o
Io = 50 W.m2
I\ = ?
Io = 50 W.m2
I\ = ?
a) (5 points) Determine the intensity of the light (in W/m2) after it has gone through
the last polarizer in situation A.
b) (9 points) Determine the intensity of the light (in W/m2) after it has gone through
the last polarizer in situation B (careful!).
c) (6 points) Determine the maximum magnetic field (in T) in the outgoing light waves
in situation B.
SHOW YOUR WORK!
3. Light traveling inside diamond (wavelength in diamond is 250 nm) hits a boundary with
air as shown in the diagram below. The light (ray) that reflects off the diamond/air
boundary is shown.
air (n = 1.0003)
θ
diamond (n = 2.42)
a) (5 points) Draw in the ray that goes into air (assuming that it does go into the air).
b) (10 points) Determine the maximum θ such that ALL of the light reflects off of
the diamond/air interface. (If θ is bigger than this one, some light goes into the air.
If θ is less than this one, NO light goes into the air.)
4. (8 points) Draw a principle ray diagram for the object and diverging mirror shown.
Use at least 3 principle rays. Draw in the image in the correct location.
ff
c
f
5. (18 points) Light comes off of an object, goes through a diverging lens with f1 = -80cm
and then goes through a converging lens with f2 = +40 cm. Determine the location,
type (real or virtual), orientation (upright or inverted with respect to the original image),
and overall magnification of the final image.
f1 = -80 cm
f2 = +40 cm
object
20 cm
20 cm
6. An interference pattern is produced by monochromatic light of wavelength 630 nm
from a distant source incident on two identical, parallel slits separated by a distance
(between centers) of 0.084 mm.
a) (8 points) It is found that the n = 5 interference maximum is missing from the
pattern because it coincides with the first diffraction minimum. What is the width
of each slit?
b) (5 points) Which other interference maxima should also be missing from the
pattern?
c) (7 points) This is done in a REALLY dark room, so that ALL the interference
maxima are seen (except those killed off because of diffraction). What is the
largest value of n that gives us an interference maximum that we can see?
7. (12 points) The UST telescope has an aperture diameter of 17 inches (43.2 cm). It is
focused on two alien spaceships that are 1000 m apart. The landing lights on the
spaceships are green (𝝀 = 550 nm). How close must the two ships get to the telescope
before they can be resolved as two separate ships? Assume this resolution is diffraction
limited.
2000 m
??
BONUS BRAIN BUSTER!!!
During sunset, is the sun actually higher, lower, or in the same place as it appears to be? Why?
Credit will only be given for correct reasoning. If you are able to guess the correct answer but
give no correct reasoning, you will receive no extra credit. However, if you give some correct
reasoning that would (or does) lead to the correct answer, at least some credit will be given.
________________________________________________
By signing my name above, I affirm that this test represents my work only, without aid
from outside sources. In all aspects of this course I perform with honor and integrity.