Sunset Rainbow Disk ~ Images by Rafael Schmall ( Photography) in Hungary. The top picture is assembled from narrower angle images. It shows well that there is more to a rainbow than a ring of colours.   ©Rafael Schmall, shown with permission.
                          
Atmospheric
Optics
About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed















Raindrops anywhere near the surface of a cone extending from the eye glint coloured light towards the eye and form the ring of rainbow colours.


What school textbooks do not say:

The rainbow is more than a ring of light - it is a disk.

Raindrops inside the cone, as well as those on its surface, glint their light towards the eye. But..     ..drops inside the cone glints many colours.   At right a drop just inside the cone is glinting blue at the most intense minimum deviation condition.   It also sends out at the same angle two red rays and two rays of all other longer wavelength colours. The extra colours are faint but collectively they dilute the blue's spectral purity.

Deeper still inside the cone the colours glinted become more and more of similar intensity. We see a white inner disk when the sun is reasonable high and red one in the already reddened rays of a sunset.
The "Rainbow Cone"
Rays at minimum deviation are the most intense. At left the blue ray is at minimum deviation and this drop lights the blue part of the primary bow.

The drop also glints longer wavelength coloured rays at the same direction.

When the drop is well inside the rainbow it glints all colours more or less equally and we see this as white (or sunset red) light brightening the inside of the bow.