Different colors have different indices of refraction

["Thomas H. Hogan" <flzhgn@xxxxxxxxxxxxx>]

Remember those NY subway lights?
Maybe those lights are bending their way to your eyes through a lens
which
refracts the red and blue to different angles according to the
wavelength and index of refraction.

Different colors (wavelengths) bend (refract) to a different degree 
when they pass through substances that have a different index of
refraction.
(This is why lenses are coated--to prevent chromatic aberration.)

You might be interested to see a photo that I took using Rheinberg
illumination (basically concentric colored filter gels of different
colors) it is the December "Cover Photo" of Microscopy Online at
http://www.microscopy-online.com/index.html

The colors used were yellow and blue (because I wanted red-green 
color-blind people to be able to see it).  So the specimen was
illuminated with concentric cones of light owing to the concentric
circular filters I placed over the light source in the microscope.

The two colors (yellow & blue) were bent (refracted) differently
owing to their different wavelengths, when they passed through
protoplasm (higher index of refraction) vs surrounding medium (water).

This "trick" is used to make many biological specimens (oocytes) that 
are colorless quite spectacularly colored.

Anyway, I thought this might relate to those questions about the subway
lights (the blue is bent away from your view unless you are close but
the red light is bent less so it reaches your eye when you are further
away).  The same sort of thing happens with the infrared focus mark on
camera lenses: the infrared light of longer wavelenght requires more
distance to focus because it is refracted (bent) less than visible
light,
except of course, in the much maligned mirror lens!
-------------------
By the way, does anyone recall the name of the effect where one can
determine the angle of polarization of a polarizer using your eye alone?
I trained myself  to do this many years ago, but I'm out of practice.

Hank Hogan <http://www.pic.net/~flzhgn/>

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End of INFRARED-PHOTOGRAPHY Digest 142
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