Light, Sound and Waves

Glories and sun dogs

Stories from Physics for 11-14 14-16 IOP RESOURCES

The Sun’s corona, brilliantly coloured rings that appear around the star, are a diffraction effect caused by the scattering of light by water droplets smaller than 60 μm in diameter in clouds (raindrops are too large to cause the effect). The phenomenon is formed when a series of diffraction patterns undergo differential scattering. A more common phenomenon is iridescent clouds which show similar colour patterns to corona but without a circular structure.

One particularly striking refraction phenomenon is the glory, a coloured halo seen around a person’s head. Glories, or anticorona, share some properties with corona, but occur at the anti-solar point (the point on the celestial sphere directly opposite the sun from a particular point of view).

Under some conditions, a series of bright lights can be seen close to the Sun giving the impression of multiple suns, referred to as sun dogs, mock suns or parhelia. The effect tends to occur when there is a layer of high cirrus clouds in which hexagonal ice crystals act as prisms, refracting light at an angle of 22°. Sometimes, a full halo at an angle of 22° is visible round the Sun, caused by the reflection and refraction of light by ice crystals in thin high cirrus clouds; a similar effect is also sometimes observable around the Moon.

References

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