Sunlight is made of many different wavelengths, or colours, that travel at different speeds when passing through a medium. This causes the white light to split into different colours. Longer wavelengths appear as red and shorter wavelengths appear as blue or violet.
We see the colour spectrum of the rainbow as the light passes through the raindrop at different angles of approximately two degrees, from red to violet. This is not a true spectrum as the colours mix and blur throughout the spectacle. The angle of scatter from raindrops is different for everyone which means that every rainbow is unique to the observer. However, for the observer to see a rainbow, they must be in a specific position relative to the sun and water droplets -.
When sunlight passes through raindrops, the light bends, or refracts, as it enters the droplet, and then reflects off the inside of the raindrop. This happens because the water is more dense than the air that surrounds it. As it exits the droplet, the light separates into wavelengths.
Visible light is made up of various wavelengths, and each wavelength appears as a different color: red, orange, yellow, green, blue, indigo and violet. Red light, for example, bends at a different angle than violet light.
Sometimes, however, rainbows can actually form an entire circle that you can see in a plane with the right conditions. So when the light exits the water droplet, it is separated into all its wavelengths. The light reflecting back to you, the observer with the Sunlight coming from behind you, from the water droplets will appear separated into all the colors of the rainbow!
Violet will be on the bottom and red on the top. A secondary rainbow appears if the sunlight is reflected twice inside the water droplets.
Secondary rainbows are fainter, and the order of the color is reversed, with red on the bottom. Credit: Leonardo Weiss via Wikimedia Commons. Sometimes you can see another, fainter secondary rainbow above the primary rainbow. The primary rainbow is caused from one reflection inside the water droplet. This is because the wheels moving on the pavement are able to roll faster than the wheels on the grass. In the case of a rainbow, when sunlight hits a raindrop it does not move as fast through the water as it does through the atmosphere, so it bends a little.
The light then turns again as it moves out of the raindrop and back into the air at its original speed. When light hits the rain at just the right angle, it is refracted through a raindrop and into our eyes, causing us to see a rainbow.
But how does the "white" sunlight produce a multicolored rainbow? Sunlight, or "white" light, is actually made up of continuous bands of different colored light--red, orange, yellow, green, blue, indigo and violet. Each color has a different wavelength, or frequency, which refracts slightly differently when it passes from one medium to another. As a result, white light can be broken up into its component colors by being passed through certain medium.
For example, a prism can also create rainbows because the glass, like the raindrop, bends the different colors of light at slightly different angles.
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