Phy 133 - Assignment 1:
Quiz A.
2. Draw the axis. Draw a line for the mirror. (The mirror is supposed to be just a small
part of a sphere, so a line is close enough and easier to draw than a circular arc.) Draw the
object with its bottom on the axis. (The question doesn’t give its height or ask for the
image’s, so make it any height you want.) Mark the focal point. I didn’t use the ray from
the center of curvature so I left that point out. On the original for the scanned copy below,
20 cm was represented by 4 cm and so forth.
From the tip of the object, draw any two of the three rays mentioned in class. I drew one
approaching the mirror parallel to the axis. From where it hits the mirror, it goes out
through the focal point. I drew another ray approaching the mirror on a line that also goes
through the focal point. After reflecting, it goes parallel to the axis. After drawing two or
more rays, look for where they cross after being reflected. Since these don’t really cross,
you don’t get a real image. But if you extend them behind the mirror to where they seem
to come from, a virtual image is there. Measure the answer off the diagram.
B. 1. If θ is more than the critical angle for total
internal reflection, the light ray from the fish
can’t cross out of the water. Or if light is going
toward the fish, as in trying to see the stone, the
ray can’t come from outside the water. So, if h
is too small, light from the stone won’t reach the
fish’s eye.
2. For how I drew this step by step, see my solution for quiz A. The main difference is
that this is a lens instead of a mirror. So, there are two focal points instead of one and the
rays go through rather than bouncing off.
C. 1.
2.If the object is inside the focal point, as in quizzes A and B, you get a virtual image. This
time it’s outside the focal point so the image is real. Just for variety, instead of drawing the
two rays that go through the focal points, this time I used the one that hits the lens at its
center. This ray just goes straight, as if the lens was a piece of window glass.
D.
E. 1. An image is real if the light rays actually pass through it. The rays don’t pass
through a virtual image; they just seem to come from there. (I would also accept it if you
said that a real image can be projected on a screen, while a virtual image can’t. But, I like
my first answer better, because it explains the underlying reason for that.)
2.
F. 1. (Not in the order the questions were asked)
b. No. The number of crests per second arriving at one side of the boundary equals the
number of crests per second departing from the other side. Where else would they go?
c. Yes. (Assuming they have different indices of refraction.) Since n = c/v, and since
c never changes, a change in n means a change in v.
a. Yes. Since  = v/f
2. a. The mirror should be concave, because a plane or convex mirror can’t make a real
image. (An image must be real to be projected on a screen.)
c. (Not in order again.)
b.
R = 2f = 25/12 = 2.08 m