Like the title says, how does a rainbow exist twice? If we could see every electromagnetic wavelength, would we see all of those “colors” in between the first and second ring? What phenomenon causes it to happen twice and always separated by what seems to be a fixed amount/distance from each other when they happen?
In: Physics
The geometry of reflections inside a rain droplet mean that a rainbow is projected in two different directions, but one is dimmer.
From the drops at a certain height, we see the first rainbow. From drops at a different height, we see the second projection, forming the second rainbow.
It’s like when you look into a mirror, there aren’t really two of you.
I don’t have anything to add beyond what other people have said… but if you’re interested, Walter Lewin, the famous MIT physics lecturer, gives a great video explaining how rainbows work. It assumes a high school-ish level of physics and math understanding.
A normal rainbow is caused by the light being refracted as it enter a water dropplet, reflects off the back of the water dropplet and then refracts again when exiting the dropplet. Because of the shape of the water dropplet and the different refraction angles of different frequencies of light the different colors gets reflected in different directions. In a double rainbow the second weaker rainbow is caused by the light getting reflected three times inside the water dropplet instead of just once. This gives a different reflection angle then when the light only gets reflected once so you get two distinct rainbows, one weaker then the other. The second rainbow is technically always there but because it is weaker then the first it is not always visible in all light conditions. There can even be three rainbows in very rare lighting conditions when you see light reflected five times inside the water dropplet. The light reflects an even number of times, zero or two times, creates a rainbow towards the sun and forms rings around the sun.
Rainbows exist because light is being reflected inside water droplets. Entering and leaving the water bends the light by different amounts depending on its wavelength, which causes the wavelengths to separate apart. But each wavelength is always at the same angle, because all the drops are the same shape (a sphere), so you consistently see each color coming at you from a different place.
The second, dimmer rainbow that makes a “double rainbow” is simply light that has been reflected twice inside the droplet, instead of once. The same principles apply, it’s just a second angle for each wavelength to come from, creating a whole second rainbow. It’s not a continuation of the same rainbow, it’s a separate one entirely. There is a fixed distance between them because the angles of both of them are fixed – primary rainbow is ~41 degrees from opposite the sun and secondary is ~52 degrees from opposite the sun.
Rainbows do extend in non-visible light too, but not forever – water is opaque to most light.
All rainbows are actually double rainbows, but the second rainbow is often very faint. This happens because of how light refracts in water. To get light going in, splitting and coming back out as a rainbow, you need a very specific angle of reflection. Several of these angles exist, but every angle between them is dead – it doesn’t output a rainbow. This is because the light coming out of the raindrops is coming out in an angle that the observer isn’t seeing. The first and second rainbows are the result of two rainbow-creating angles being hit by the sun, and the empty space between them is space where the angle of refraction is dead.
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