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OPTICAL PHENOMENA
Rainbow
Rainbows are one of the most well-known optical phenomena that occur in the atmosphere. A rainbow is formed through two processes, refraction and total internal reflection of sunlight.
When light passes through a transparent substance such water, it is bent at a slight angle, or refracted. Because colours of different wavelengths are refracted at different angles, the colours of the spectrum become separated.
Sometimes light enters a transparent object at one end, but does not pass out of the object when it reaches the other end. Instead, it is reflected back into the object again. This is called total internal reflection and occurs when the light hits the end of the object at certain angles. |
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In a rainbow, rays of sunlight which strike raindrops in the sky, are refracted through them, separating into their constituent colours, and are then totally internally reflected off the backs of the raindrops and refracted out again. Each colour emerges out again at a slightly different angle. Because the same colour will emerge in the same way in every one of the millions of raindrops, the observer will see distinct bands of colours. In effect, the raindrops act like millions of tiny prisms, breaking up the sunlight into its component colours and then reflecting these back toward the observer.
As rainbows are formed by the interaction between sunlight and raindrops, they therefore usually occur when it is raining. To see a rainbow, the observer has to be between the sun and the rain, facing the rain and with his back to the sun.
A rainbow forms its largest arc when the sun is close to the horizon. The higher the sun in the sky, the flatter the rainbow will be.
Halo
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A halo is a coloured or whitish ring or arc of light around and extending outwards from the sun when seen through cirrostratus clouds which compose of ice crystals. It is caused by the refraction and reflection of light by the ice crystals in the cloud acting as tiny prisms or mirrors. When sunlight passes through these crystals, the rays of light are dispersed in different directions depending on the wavelength of light, with blue light bending more than red light. |
A halo seen in Singapore on 28 Aug 2010
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Solar halos are generally observed in the presence of cirrostratus which is transparent whitish and fibrous clouds that are composed of small hexagonal ice crystals. It is difficult to forecast the presence of these ice crystals in the atmosphere, and the sighting of such phenomenon depends on the vantage point of the observer.
Although it is safe to look at the halo itself with the naked eye, it is dangerous to look at the sun directly without proper protection. The cardinal rule in solar observation is safety. Never look directly at the sun with the naked eye - this may cause total blindness within seconds. The focused light of the Sun from even a small telescope or a camera contains enough heat and light to do permanent damage to your eyes from even the briefest exposure.
For more educational information on halo and other optical phenomena, Singapore Science Centre has a very informative web site at http://www.science.edu.sg.
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