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why in formation of  rainbow raindrops undergoes total internal reflaction then refraction
 

ashok singh , 10 Years ago
Grade 12
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Aarti Gupta

Last Activity: 10 Years ago

Rainbow is an optical and metereological phenomenon which involves a series of physical phenomena called reflection, refraction, dispersion and total internal reflection respectively.The occurence of each of these is because of the interaction of light with air and water and the boundaries between them.Whenever light from the sun hits a water droplet,some of the light is reflected and such light will obey theLaw of Reflection.Now the light that is not refracted crosses the air-water there is reduction of speed cause the path of the light to bend and this is calledrefraction.In this case the path of the light rays bendstowardthe normal line.White light is made up of a spectrum of colours,each with its own wavelength.Different wavelengths travel at different speeds and when they encounter a change to medium that is more dense or less dense,their speeds are efected by different amounts.Hence,the colours separate.This phenomenon is know as dispersion.At the rear of the raindrop,the light hits the water-to-air interface.If the angle of incidence is greater than the critical angle,then total internal reflectionwill occur. A rainbow will only be seen if this happens, otherwise the light will continue out the other side of the raindrop and continue to move away from the would-be viewer.
Just as the light changed speed as it entered the raindrop,its speed changes again as it leaves.Here, the light is moving from a more dense medium (water) to a less dense medium (air).As it does so, it speeds up and its path bends.In this case the path of the light rays bendsaway fromthe normal line.This is another example ofrefraction.As the rays are refracted once again, the various wavelengths are effected to different extents.The overall result of this is increased separation of the component colours of white light which is called as dispersion.

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