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Glow formation
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   Zero order glow formation


Most of the sunlight entering raindrops leaves the other side without internal reflection. Only a small fraction is reflected to form the primary, secondary and higher order rainbows - in the diagram these rainbow forming rays are shown very faint.
No internal reflection  
  As the distance of the incident ray from the drop centre increases, the outgoing ray deviation increases continuously . There is no turning point or angle of minimum deviation at each side of which the deflection changes in the same direction. This is why the zero order rays do not produce a rainbow.

No turning point, no rainbow

Contrast the zero order behaviour with that of the primary bow rays. In the latter the deflection decreases to a minimum - the rainbow angle - and then starts to increase again as the incident ray's distance from the drop centre is continuously increased.   Different behaviour from rainbow rays
  Refraction disperses the colours but they overlap again outside the drop. The glow is the same colour as the incident sunlight.

Colours recombine
  The brightness of the zero order glow is one of the causes of the difficulty in ever seeing 3rd and 4th order rainbows outdoors.

Bright sky inside the bow


Straight through rays.

Mouse over the slider for the ray paths. There is no turning point at an angle of minimum deviation and so rays produce a diffuse glow rather than a rainbow.