|Boomerang Optics imaged last week by Keith Webb over the New South Wales Riverina, Australia. ©Keith Webb, shown with permission.
The primary rainbow extends well below the horizon.
With luck, a complete or almost complete circular rainbow can be seen from a relatively low flying airplane or from a mountain. However, it is not easy to see or photograph through an aircraft window as the complete bow is a huge 84° in diameter centered at the antisolar point.
Rainbows are produced by drops 0.3mm diameter and upwards. Smaller cloud droplets produce another antisolar point phenomenon - a glory. Light is scattered and diffracted almost directly backwards towards the camera.
In the right hand image from the Boomerang's return flight the shadow of the photographer's airplane is at the glory centre. The accompanying aircraft does not have a glory but its pilot would have seen the situation reversed.
Other antisolar optics are the heiligenschein and opposition effect.
The heiligenschein is a soft glow produced by water droplets focusing sunlight on leaves or the ground and then imaging it backwards to the eye.
The opposition effect is partly an apparent bright area where shadows directly opposite the sun are hidden behind the objects (trees, stubble) hiding them.
The glow of one or other (or both) effects is visible in the lower image. Again, only the photographer's aircraft has the halo like glow. This view is contrast enhanced. The effects are often best seen visually when you are moving. The low contrast patch of lightness then becomes more apparent as it moves across the landscape.