When a beam of sunlight L strikes upon a raindrop at A (Fig. 1),the light that passes into the drop is refracted at A and dispersion begins at the same place, so that when the light strikes the back of the drop it is as a spectrum, with the red ray above on account of its refraction being the least. At RV part of the light passes out of the drop, but part is reflected. Since the angle of incidence ARO is greater than the angle AVO, the angle of reflection ORR' must be greater than the angle OVV', and for this reason the rays will cross at B and emerge (in part) from the drop at R' and V' in the directions R'R" and V'V" The other spectrum colors will be refracted in regular order between R" and V".
FIG.1
The angle of in cadence at which these results are obtained is such that the red ray leaves the drop at an angle of 42° to the direction of the entering ray, and the violet ray at an angle of nearly 40°. Hence the red ray is seen at an angle of 42° and the violet at an angle of about 40° to a line drawn from the observer
FIG
directly away from the sun. Since the angle for the red ray is greater than for the violet, it is evident that if the eye is placed at E (Fig. 2) the different colors will be seen reflected from different drops, and that the drop giving the red ray will be higher than the others. This explains why the red is on the outside of the primary bow.
Demonstration. - By using a thick cylinder of glass with the optical disk, the path of a ray similar to that which forms the primary bow in a raindrop can be traced as represented in Fig. 3.
