square mirrors, round dots, why?

[Spearmint Rhino]

You know how a mirrorball is made up of square tiles, right? How come the dots of light it throws around the room are circular?

[Alderman Barnes]

square mirrors, round dots, why?

Because the lamp shining on it is round?

[Spearmint Rhino]

square mirrors, round dots, why?

Well, the lamp in this case being the sun, but OK.

However, if I put my rectangular make-up mirror in the sun, and angle it so it throws a reflection on a nearby wall, the reflection is a quadrilateral.

So, presumably, there's a certain distance (between reflective surface and the final 'screen' surface) at which quadrilateral reflections flip into circular ones. Right?

[Alderman Barnes]

square mirrors, round dots, why?

You know, I haven't really thought this through, have I?

[Stumpy Pepys]

square mirrors, round dots, why?

I'm reluctant to join in, as I suspect the correct answer is probably bastard complicated and involves interference, quantum theory and allsorts.

[Spearmint Rhino]

square mirrors, round dots, why?

I've realised I've inadvertently revealed loads about my lifestyle in this simple physics question, with details such as 'mirrorball' and 'make-up mirror'...

[delicatemoth]

square mirrors, round dots, why?

Dunno about 'revealed' - I suspect your lifestyle is not half as glamorous as you've just made it sound...

[Guest]

square mirrors, round dots, why?

I read about the Mpemba effect yesterday, "named for the Tanzanian high-school student Erasto B. Mpemba. Mpemba first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School"

http://en.wikipedia.org/wiki/Mpemba_effect

And that Aristotle, Francis Bacon and René Descartes have all claimed that hot water does freeze more quickly than cold water.

Nothing will surprise me anymore.

[Wyatt Earp]

square mirrors, round dots, why?

This is a good one. I don't know the answer, but my guess is it's the same as the reason why the dapples in sunlight under a tree are circular: that is, the mirrors are small enough, compared to the distance to the wall, to act like a camera aperture.

Each point on the light source is associated with a square reflection, but when you put together all the points you get loads of square relections smeared over a disc shape. If the disc is small compared to the mirror, the reflection will still look square-ish, though perhaps with rounded corners. But if the disc is large compared to the mirror, the mirror's shape is lost and what you see is just a disc.

Try drawing a circular smudge using the coarse pen setting in Paint, and you might get a feel for what I mean.

[Alderman Barnes]

square mirrors, round dots, why?

Do you remember that partial eclipse we had last year? I happened to be sitting next to the window and saw rows of little crescent-shaped suns shining on the desk through the holes in the blinds that the cords go through.

If you tried that with a mirror ball it would blow your mind, I reckon.