The Span of a Rainbow

September 27, 2014


Photographers: Top Photo - Bonnie Sizer; Bonnie's Flickr site; Bottom Photo - Phil Thomson
Summary Authors: Phil Thompson, Bonnie Sizer, Kevin Stewart, Jim Foster

Shown above are two magnificent rainbows. The top one (a double bow) taken from Boulder, Colorado, near sunset on the evening of August 27 and the bottom one taken from Barrabool, Victoria, Australia, in the afternoon of July 10, 2014. Both primary rainbows span the same amount of sky, but the Boulder bow arches higher that the Barrabool bow since it was taken closer to sunset.
Sometimes a rainbow appears to be right in front of us, but there's really no specific distance from an observer where a rainbow can be exactly located. Because 35 mm cameras have a field of view smaller than the angular size of the primary rainbow (42 degrees), attempting to take a bow’s picture can be an Image001exercise in frustration. The best way to capture the entire bow is to use a fisheye or wide-angle lens or to make a montage of several shots.
As shown at left these two rainbows, and in fact all primary rainbows, have exactly the same radius from the antisolar point. However, because the Barrabool rainbow was seen earlier in the day, when the Sun was higher, it thus covers much less of the sky than does the rainbow observed over Boulder -- about 60 percent less.

Photo details: Top - Camera Model: NIKON D5100; Software: Adobe Photoshop CS6 (Windows). Bottom - Camera Model: PENTAX K-5; Lens: Smc PENTAX-DA 12-24mm F4 ED AL [IF]; Focal Length: 15mm (35mm equivalent: 22mm); Aperture: f/14.0; Exposure Time: 0.010 s (1/100); ISO equiv: 100; Software: ACDSee Pro 7.