The y coordinate of the incident light can be changed when clicking a mouse in the
area of the upper half of the drop of water and doing a drag.
The angle between the incident light and the refracting light is maximized in the
point that a dot has been put on the left. Even if the position of the incident
light changes in the neighborhood of this point, the direction of the refracting
light doesn't change too much. Therefore, the rainbow can be seen in this direction.