Atmospheric
Optics
About - Submit Optics Picture of the Day Galleries Previous Next Today  
                       
Mystery Bow ~ Jos Nijland in The Netherlands pictured this combination of bows from distant falling rain and a nearby tulip field sprinkler. See here for more on tulips in Holland and 17C tulip mania. The colours of the sprinkler bow do not match those of the rainbow – something odd is happening.       Images ©Jos Nijland, shown with permission
The purple-green bands at right are supernumeraries. They are prominent because monosized drops were used in the calculation.



At right some accurate Mie theory computations of bows for different size drops.

The bow broadens as drop size decreases. Yellows, greens and blues shift inwards.

Colours weaken as the drop size decreases. The 0.2mm diameter droplet bow is a white - straw colour and compares with that at the top of the sprinkler spray.

OPOD repeatedly says that bows from distant and nearby rain are the same and that both are illusions apparently at infinity. Is this an exception?

The colour enhanced version of the tulip sprinkler bow (right) is clearly different from that from the distant rain. Yellows and green of the sprinkler bow are moved towards the right. The colour bands are also tilted in a different direction. The top of the windblown spray has no definite colour at all.

Differences in the size of the drops making the bows can cause these effects. Spray is being carried away sideways from the main sprinkler jet. Very small droplets will drift almost horizontally. Large drops will sink down quickly close to the jet. Across the spray there will be a gradation of droplet sizes small at the top and becoming larger lower down and closer to the main jet.

All this changes the appearance of the sprinkler bow.