On Mon, 2007-10-22 at 17:33 -0500, Charles Henry wrote:
On 10/22/07, Martin Peach martin.peach@sympatico.ca wrote:
Mathieu Bouchard wrote:
A very simple way to explain aliased frequencies would be: spin a bicycle wheel. When you accelerate it beyond a certain point, it will begin to look like it's going backwards instead. This is because the wheel speed, together with the repetitiveness of the wheel's appearance, have crossed the Nyquist frequency of your eye.
That won't work in sunlight for example.
Haven't you ever seen it? (in sunlight that is)
You need a flashing light source like a fluorescent tube. It's the Nyquist frequency of the light source that causes the aliasing, not of your eyes, unless you try it while blinking rapidly. The wagon wheels in western movies appear to turn backwards when the spokes are moving faster than the frame rate of the movie.
Martin
Vision doesn't work exactly like a camera.
i think, that is the point that martin is trying to make.
i am not an expert either, but when i encountered this effect (watching the frontwheel, while cycling at night, for instance), there was always artificial light involved. to be more precise, there was always kind of a fluorescence or any other kind of 'gas discharge lamp' (is that the correct expression?) involved. it doesn't seem to work with common light bulbs, since their 'afterglow time' is too long.
for me, it seems, that it is rather related to the light source and its frequency and not to a property of the eye.
roman
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