Re: parabolic / sperical mirror : How to tell

Depends upon the focal length of the mirror. You said that the image of the
sun was about a dime in size so it musn't be that much.
A parabolic mirror iwll make a more accurately focuse imag eof the sun. The
problem with using the sun is the reflected image will be so bright that you
can't look at it. Better to take it out at night and set it up on a stick
so you can put an eyepiece (a 10x eyeloupe will do fine for this) and look
at a distant light or a star. If the image iw well focused (it will
probably have a blur in one direction as you aren't on the axis of the
paraboloid) then you probably have a paraboloid. A spherical mirror at a
short focal length will have a very soft focus with a distant object.
Another test is to take a LED and put it behind a straight edge (hopefully
of a thin metal) and put it at the radius of curvature of the mirror and see
what the shadows look like when you start blocking the light (your basic
Foucault test). You will be wanting to hold the mirror in a stand so it
doesn't move and then have the LED/KE on another stand so things don't move
aobut a lot. Look up the Foucault test in google to see what is happening.
The test isn't hard to do even with even primitive wood equipment for the
parts.

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Why do penguins walk so far to get to their nesting grounds?


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