Color vision and chromatic aberration:

One of the remarkable ability of the eye is its ability to see color.

There are three types of cones that respond to light from three different parts of the spectrum .
-    Images on a color TV screen are produce by a method similar in some aspects.
-    If you examine a color TV screen with a magnifying glass , you will see a myriad of small red ,green ,and blue dots .These dots can produce in different  combinations all of the colors of the spectrum.
-    If is thought that in an analogueway ,signals are sent to the brain from three colored cones in different combinations permitting it to determine color.
-    If one color sets is gone , a person is color blind – he confuses certain colors
-    Approximately 8% of all man and 0.5% of all women are color blind .

Chromatic aberration :

Is a common lens defect caused by the change of the index of refraction with wavelength.(This permits a prism to separate white light into a rainbow of colors).

-    Chromatic aberration causes different colors to come to a focus at different distance.
-    Chromatic aberration is greatly reduced in an expensive camera because a lens is used that made of several types of glass chosen to compensate for the change of the index of refraction with wavelength .

Purkinje effect:

A special color effect that is sometimes noticeable at dusk,Purkinje noticed that at dusk the blue blossoms on his flowers appeared more brilliant than the red blossoms.

-    This effect is caused by the shift of the best sensitivity of the eyes toward the blue as the rods take over from the cones at two light levels.
-    Since the eyes and corrective lenses are optimized for yellow light ,this shift toward the blue produces a refractive error of about 1.0 D, in  other words ,for night vision you should wear a glasses with an additional 1.0 D.