The light of the sun is in fact
composed of all colors,
purplec red, blue, green, yellow...
The addition
of all this colors gives
the white light.
When light penetrates in
a drop of water it acts as
a prism, I mean that the
components of the solar light
are going to be deviated while
passing air
to water, but not all of the same
way: the red will be deviated little,
the yellow a few more, etc, and the purple will be very deviated.

To simplify, the sun rays arrive in A. At the time of penetrating in the drop of water the ray is bent (refracted) and go hit the internal surface of the

drop in B where he is reflexive toward C. While coming out of the drop he is refracted again and comes to hit your eye in D.

Descartes write :  " [.] having taken the pen and calculated all rays that fall on the various points of a water drop, to know under what angles, after two refractions and one or two reflections, they can come toward our eyes. I found that after a reflection and two refractions most of the rays can be seen under the angle of 41 to 42 degrees. "

Descartes means that the sun rays can be decomposed in all its colors, come out of the drop of water with an angle close to 42°.

Therefore, the rainbow is a circle of angular ray of 42°, centered on the point contrary of the Sun (" antisolar " point), as shown above.