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Re: [Phys-l] Sun's image due to reflecton by not a pinhole.

I had students outside on a sunny day with round and/or rectangular mirrors reflecting sunlight onto the brick wall.

Some wondered why square mirrors made a round spot of light.

Some didn't care (of coarse).

If the 'screen' is CLOSE to the mirror - the 'spot' of light is square.

Is this 'correctly' viewed as near-field and far-field domains??

(clearly, Jackson made an impression - but not a very useful one ;-)

On Apr 29, 2009, at Apr 29(Wed) 11:53 , Bernard Cleyet wrote:

Intuitively, I think a small plane mirror will act "like" a pin
hole. A member of tap-l claims that a large mirror (eight inches
diameter) will do the same if the "focal" length is long enuf.
(logically meaningless). He then backed it up by experiment. i.e.
nice image at 50 feet.