Quadruple Rainbow

Not until this year has a triple rainbow or a quadruple rainbow been photographed and published in the scientific literature.
Quadruple Rainbow
Such rainbows are more technically referred to as tertiary or quaternary rainbows. That makes it well nigh impossible to capture all four rainbows in the same picture — and because some light is lost with each bounce, the third and fourth rainbows are incredibly faint. If the sun broke through the clouds under these conditions, it could project a dim tertiary rainbow against the dark clouds nearby, they said.
Quadruple Rainbow

Michael Grossmann / Applied Optics

A processed version of the image is at right, revealing a faint tertiary rainbow between the white arrows.
Quadruple Rainbow
Some experts thought it'd be impossible to make out the rainbow, but amateur rainbow-chasers rose to the challenge. advertisement

I decided to use that technique to increase the chances to record the third-order rainbow. I took several image series until the rain stopped at my location. I did not see that rainbow visually.
Quadruple Rainbow
"Back home I started processing, and already the first image series that I took when the sun brightly lit the raindrops showed the third-order rainbow! I checked the Internet for higher order bows and quickly realized that that image series likely showed the fourth-order rainbow.
Quadruple Rainbow
Theoretically, it's possible to have a quintuple or a sextuple rainbow, but the optical geometry of the bounces within the raindrop is such that the fifth- and sixth-order rainbows would be overwhelmed by the light from the first- and the second-order rainbows. The research papers describing the observations, and providing guidance for future rainbow-chasers, appear in a special issue of Applied Optics.

Source : http://photoblog.msnbc.msn.com/_news/2011/10/05/8173218-whoa-its-a-quadruple-rainbow