Q. What is the cause of a rainbow?
A. When the clouds opposite the sun are very dark, and rain is still falling from them, the rays of the bright sun are divided by the rain-drops, as they would be by a prism.
Let A, B, and C be three drops of rain; SA, SB, and SC three rays of the sun. SA is divided into the 3 colours; the blue and yellow are bent above the eye D, and the red enters it.
br /> The ray SB is divided into the three colours; the blue is bent above the eye, and the red falls below the eye D; but the yellow enters it.
The ray SC is also divided into the three colours. The blue (which is bent most) enters the eye; and the other two fall below it. Thus the eye sees the blue of C, and all drops in the position of C; the yellow of B, and of all drops in the position of B; and the red of A, &c.; and thus it sees a rainbow.
Q. Does every person see the same colours from the same drops?
A. No; no two persons see the same rainbow.
To another spectator the rays from SB might be red instead of yellow; the ray from SC, yellow; and the blue might be reflected from some drop below C. To a third person the red may issue from a drop above A, and then A would reflect the yellow, and B the blue, and so on.
Q. Why are there often two rainbows at one and the same time?
A. In one rainbow we see the rays of the sun entering the rain-drops at the top, and reflected to the eye from the bottom.