Water Drop Reflections and Multiple Rainbows

April 11, 2011

Photographer: Michael Grossmann 
Summary Author: Michael Grossmann; Jim Foster

A rainbow is a product of millions of falling raindrops interacting with sunlight. With but a single reflection of sunlight inside a water drop, a primary rainbow will form; with two reflections the secondary bow is visible.  However, under ideal conditions, there can be many more orders of reflection. As shown above, five, six and even ten internal reflections can be observed. Moreover, it's theoretically possible to detect twenty internal reflections, but the problem is to produce a perfectly spherical water droplet. The drops I used for this experiment were formed artificially. The light source is a 5 mW green laser pointer. Note that the bright spot at left center is the laser-illuminated water drop.

The third and fourth-order reflections aren't shown here because they, along with the seventh and eighth order reflections, are positioned on the other side of the picture  -- in the direction of the light source. To better understand this setup, imagine that you're standing with your back to your computer screen and the Sun is shining behind you -- over the screen. The primary and secondary bows will be viewed in the direction you're facing -- opposite the Sun. The fifth, sixth, ninth, and tenth order reflections are also in this direction. However, the third and fourth (as well as the seventh and eighth) order reflections can't be seen because they're behind you. Click here to see a model of the experiment set up.

Under exceptional atmospheric conditions it may be feasible to see the third and fourth-order bows if you're facing the Sun, but they're quite faint. A third-order bow, for instance, is one quarter as bright as a primary bow. A fifth-order rainbow is only about one-tenth as intense as the primary bow.