At the End of the Rainbow

Explanation

Light refraction in a raindrop - Rainbow

To answer the question how the colours of the rainbow become visible, we have to examine the path of the light through the raindrops. The light is reflected at the back of the raindrop. Both at entering and at leaving the raindrop, the light is refracted. The individual colour components are refracted equally, however. For example, the blue components are refracted more than the red ones; they leave the raindrop at an angle of 40°, but red ones at 42°. This causes the white sunlight to be fanned-out into its spectral colours at the raindrop.

Move your cursor over the image to highlight two example raindrops.

Set-up of the sun, the observer, the rainfall and the the rainbow.

But a rainbow is visible only when millions of raindrops refract and reflect the light as described above. Although every raindrop reflects the whole light spectrum, we see only one colour per raindrop. The colour components leave the raindrop at specific angles. Our eyes are reached only by the colour that leaves the raindrop at the according angle. Depending on the position of each raindrop, they reflect a specific colour into our eye so that we always see rainbow in the colours red, orange, yellow, green, blue and indigo.


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