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## Class 12 Physics (India)

### Course: Class 12 Physics (India)>Unit 9

Lesson 7: Some natural phenomena

# Formation of rainbows (& how far are they)

Rainbows are formed by the dispersion of light & reflection (not total internal reflection) from drops of water. The rainbow color sequence can be analyzed by drawing a ray diagram of the refraction of sunlight inside water drops. Created by Mahesh Shenoy.

## Want to join the conversation?

• Why do the rainbows look like the bow (why they are curved the way they are)?
• Look...The rainbows are a full circle coz the raindrops are round, but due to the horizon, we can only see half the circle...The Horizon Blocks the other half..
• Why is total internal reflection not possible inside raindrops?
• Say the light hits the drop making the angle A with the normal; note that 0°<=A<90°. It bends toward the normal, so it enters at angle B (found with Snell's Law) where 0°<=B<C, and C is the critical angle.

Now look at the ray diagrams. The normals all contain a radius of the drop. The path taken by the ray inside the drop makes an isosceles triangle with these radii, meaning it hits the back surface making the same angle B<C. Since total internal reflection only happens when B>=C, it is impossible.
(1 vote)
• I have query that what will be the rainbow phenomena different in heavy rain and gentle rain . Also if the time is afternoon or dawn ?
• I look at rainbow picture and found that there are actually two semicircle, in this lecture we talked about inner rainbow but not about another bigger rainbow, I am very curious how the bigger semicircle rainbow formed and there colors are symmetry too!
• At , what will happen if all of the light leaves the droplet? And if a rainbow is formed, it is formed with a collection of droplets. But the droplets fall to the ground, How does the rainbow stay there?
And last question why don't we see rainbows close to the ground?
• If all the light leaves the droplet ( as in gets refracted by second side of droplet to the outside air), then you will not be able to see a rainbow because only the light that comes back to our eyes after refraction make us perceive the rainbow. A rainbow is visible even after rain because some amount of moisture is still suspended in air and it will disperse and refract light. Rain drops aren’t necessary for rainbows to be seen. Rainbows can also be viewed over waterfalls or fountains- any place that has condensed water droplets suspended in air and sunlight opposite the water droplets.
• If the light is refracted backwards then why don't we see the rain behind the raindrops?
• This is by far the BEST explanation. Thankyou!
• So is there only one droplet that reflects the light to its own back (like shown above) , and then the light passes out and is double- refracted by all the other drops(passes through) ??
If yes, what will happen if that one drop reflects only one color and the rest exit the drop.
If not, what will happen if all those drops reflect only one color and the rest exit the drop?
(1 vote)