Re: Optics & Optical Illusions

DGoncz_at_aol.com
Date: 02/11/05 

Date: Fri, 11 Feb 2005 20:48:33 +0000 (UTC)

Dear Richard,

You wrote that the blue sphere, which is larger, appears smaller from a
distance. I suggest this is due to dispersion. In general, shorter and
more energetic wavelengths of light are refracted more strongly by
lenses and prisms than longer, less energetic light waves.

From:

http://micro.magnet.fsu.edu/primer/java/prismsandbeamsplitters/equilateralprism/index.html

"Refractive index is often dependent upon the wavelength of light, with
shorter wavelengths (blue light) being refracted at greater angles than
longer wavelengths (red light). This variation of the deviation angle
with wavelength is referred to as dispersion, and is responsible for
the phenomenon that Newton observed over 300 years ago."

It seems at a distance, the angle subtended by the ornament is small,
making the dispersion apparent. When the viewer is closer to the
ornament, this angle is large, and while dispersion will distort the
perception of the red and blue sphere sizes, it will not distort them
enough to cause an obvious misperception.

To test this idea, try to find the viewing distance at which the red
and blue spheres appear the same size, and explore small changes in
viewing distance from this point. I'm pretty sure this will be most
noticeable with blue and red, which are most widely separated, rather
than with red and orange, or green and blue.

I am fairly sure that the Purkinje effect, which has to do with the
sensitivity of the receptors to blue and red and low and high
brightnesses, is not relevant. That is named after Purkinje's
observation that at dawn, the mile markers on the canals in Holland,
which are red and blue, have apparent brightness the opposite of the
way they appear during the day, if memory serves.

You have my permission to make use of what I have written in any way
you'd like. I'd suggest if you quote that web page I've quoted from,
you supply a reference.

Just one thing, what does it mean when you write that in twilight the
red lights are visible "sooner" than the blue? Do they blink? Are you
talking about sequencing or the point during the day when the lights
are overwhelmed by ambient illumination?

Yours,

Doug Goncz
Replikon Research
Seven Corners, VA

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