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Re: Physics Errors (was"Peanuts" Cartoon of 12-7-98)



I must admit that I did not read the sentence carefully.

"objects in the water dish are closer than they appear."

Being lazy my brain assumed that the cartoon referred to the well
known illusion. Some of us are obviously more critical than others,
especially outside of the regular learning situations. That is
good. Extra training in the art of critical thinking from
somewhere?

Maybe some of us write a lot of optics exam questions.
Eg: my PHY112 final exam this coming May:

...show the cartoon...

This cartoon uses the legend commonly etched in a car side view mirror. A
similar joke was used in "Jurassic Park." Address the following questions:

1a. Automobile side mounted, rear view mirrors are usually CONVEX.
Why is this? Justify your argument with a roughly sketched ray diagram
that includes the car and the area behind the car. (Hint: Why is a spherical
mirror superior to a plane mirror, and why is convex better than concave?
Assume Do>>R.)

1b. What are the image characteristics of a convex side mirror? How
is the image placed, what are reasonable magnifications, is the image real or
virtual? What geometrical image distortions are produced by this mirror?
Justify your claims with a ray diagram, and with reference to the Gaussian
mirror formula. Explicitly state the restrictions on your claims.

1c. Is the etched warning meaningful? How so (or not so)?


2. Assume the water dish in the cartoon above is empty, and you are looking
into a roughly spherical CONCAVE stainless steel bowl. Describe the image
characteristics. Is the image really closer? Justify your claims with a
ray diagram, and with reference to the Gaussian mirror formula. What
restrictions apply to this situation?

2b. What should the warning on a CONCAVE auto side view mirror read
(if such things existed)? What geometric distortions does this mirror
produce?


3. Suppose the water dish is in fact made of dark opaque plastic, and all
reflection is from the PLANAR air/water interface. Where is the image
placed, and what are its characteristics? Would the original warning be
accurate? Justify your claims with a ray diagram, and with reference to
the Gaussian mirror formula. (Hint: the radius of curvature of a plane
mirror is infinity.)

4a. BONUS: If the warning were to be etched on the bottom of the water
dish and viewed through a depth of water D, what should it read? Does
such print appear closer or farther than it really is?

4b. You are hired as a dishwasher for the neighbourhood greasy spoon.
Your first task is to drain an industrial-sized sink full of cold, greasy water.
It APPEARS that the plug at the bottom is barely within reach of your armÕs
length. Should you go for it (reach in and grab it)? Why or why not?
(Hint: Consider part A of this question.)

Hey, this is as good as Piggy's glasses for a final intro optics exam question!
(And more fun than the publish-or-perish, meet the statement of expectations
stuff I have to get back to... :^)

Happy Holidays,

Dan M

Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
danmac@nau.edu http://purcell.phy.nau.edu PHYS-L list owner