Re: [Phys-l] rainbow

[Brian Whatcott <betwys1@sbcglobal.net>]

At 04:11 PM 3/27/2007, Julie Quah, you wrote:

> Dispersion of white light occurs due to different refractive index of each
> coloured light. How does that explain the different rainbow colours
> observed? Is it because each light enters the eye at different time?

The refractive index is a convenient way of describing how media
bend light rays. It's quite a trick to prevent a lens from bending
 light rays of different colors (and hence frequencies) by different
 angles.  Newton noticed that an ordinary prism did split a ray of
 sunlight entering his digs at the window,  by different angles
and so into various colors.

You will notice that diagrams of eye sensitivity cover a color band
of frequencies in a range of less than 2:1. We do not see light of
 frequencies lower than the deep red, but we can still feel the
 heating effect. We cannot see light far past the ultra blue
but we can feel its heat and see the reddening and tanning
 it provides our skin.

It's an interesting question to ask how the eye distinguishes
 lights of various colors.   The color sensors we use for day
 vision can respond to light centering on three frequencies
 (and colors). The handling of these inputs in the optical
 processing part of the brain  uses ratios of two or three
of these sensor signals to deduce the color - though this
 is an oversimplification of the pre-processing that in
fact occurs.

Animal studies demonstrated that there are signals responding
to lines of particular angles, moving in particular directions
 and so on....

In general, light of different colors can arrive at the eye
together and still be associated with a color



Brian Whatcott    Altus OK    Eureka!