The rainbow halo seen around a plane's shadow on a cloud is called a "glory" or "pilot's glory," an optical phenomenon caused by the backscattering of sunlight from tiny water droplets in the cloud.
Here's a more detailed explanation:
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What it is: A glory is a circular rainbow-like effect that appears directly opposite the sun from the observer. In the context of an airplane, the observer is inside the plane, and the glory appears around the plane's shadow projected onto the clouds below.
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How it forms:
- Sunlight: The sun shines on the clouds below.
- Water Droplets: The clouds are composed of countless tiny water droplets, usually of uniform size.
- Backscattering: When sunlight encounters these water droplets, it's bent (diffracted) and reflected back towards the light source. This process is known as backscattering. The smaller and more uniform the droplets, the more distinct the glory appears.
- Interference: The backscattered light waves interfere with each other, creating constructive and destructive interference. This results in the colored rings that make up the glory. The innermost ring is usually bluish, and the outer rings are reddish, though the colors can be less distinct than in a typical rainbow.
- Shadow's Role: The glory appears centered around the shadow of the airplane because the backscattering is most intense in the direction directly opposite the sun. The shadow serves as a visual reference point for the center of the glory.
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Why it's special to pilots: Pilots are often in the ideal position to see glories because they're high above the ground, with clouds below and the sun behind them. This is why it's sometimes called a "pilot's glory."
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Similar Phenomena: Glories are similar to other optical phenomena like coronas (which appear around the sun or moon) and heiligenschein (which appears around the shadow of your head on dewy grass). All of these are caused by the diffraction and backscattering of light.
In summary, the rainbow halo on a plane, known as a glory, is a fascinating optical illusion caused by the interaction of sunlight with water droplets in clouds, creating a colorful ring around the plane's shadow.
