Rainbows - Zero order glow formation

Last updated on September 16, 2023

Rainbows - Zero Order Glow Formation

Rainbows are one of nature's most captivating optical phenomena. They occur when sunlight is refracted, or bent, as it passes through raindrops in the atmosphere. While we often associate rainbows with vibrant bands of colors, there is another phenomenon that can be observed alongside the main rainbow - the zero order glow formation.

Understanding Zero Order Glow

When sunlight enters a raindrop, most of it passes through without any internal reflection. However, a small fraction of the light is reflected and dispersed to form the primary, secondary, and higher order rainbows. In contrast, the zero order rays do not undergo internal reflection and instead produce a diffuse glow.

Ray Behavior and Deviation

The behavior of the zero order rays differs significantly from that of the rainbow-forming rays. As the distance of the incident ray from the center of the raindrop increases, the outgoing ray deviation continuously increases. Unlike the rainbow-forming rays, there is no turning point or angle of minimum deviation for the zero order rays.

Absence of Rainbow Formation

The absence of a turning point or angle of minimum deviation is the reason why the zero order rays do not produce a rainbow. Instead, they create a diffuse glow. This glow lacks the distinct banding and color separation characteristic of rainbows.

Contrasting Behavior with Primary Bow Rays

To better understand the behavior of the zero order rays, it is helpful to contrast them with the primary bow rays. In the primary bow, the deflection of the rays decreases to a minimum (known as the rainbow angle) and then starts to increase again as the distance of the incident ray from the drop center increases. This behavior is different from that of the zero order rays.

Recombination of Colors

As sunlight passes through raindrops and is refracted, it disperses into its component colors. However, outside the raindrop, these colors overlap again and recombine. This recombination of colors is why the glow observed in the zero order rays appears the same color as the incident sunlight.

Brightness and Difficulty in Observing Higher Order Rainbows

The brightness of the zero order glow is one of the factors that make it challenging to observe higher order rainbows outdoors. The intensity of the glow can wash out the fainter bands of higher order rainbows, making them difficult to discern against the bright sky inside the primary bow.

In conclusion, while rainbows are a stunning natural phenomenon, there is more to atmospheric optics than meets the eye. The zero order glow formation provides a fascinating insight into the behavior of light as it interacts with raindrops in the atmosphere. By understanding the distinct behavior of zero order rays and their absence of rainbow formation, we can deepen our appreciation for the complexity and beauty of rainbows.

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.

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

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Reference Atmospheric Optics

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