Many of us have seen rainbows in the sky as soon as the sun comes out after a rain shower. In order for us to see a rainbow, the conditions have to be just right.
We need some water droplets in the air – like rain or even fog – and we need the sun behind us and fairly low to the ground. This is because a rainbow is formed by light falling through water droplets.
The light coming from the sun appears white to us. But the white light that we see in everyday life is actually a mixture of different colors. When light passes through a raindrop, these colors can separate.
light waves
You wouldn’t recognize it to look at it, but light travels in waves, like waves moving across the ocean. Each of the colors in the rainbow has what we call a different “wavelength.”
This means that the distance between the peaks of the wave is different for each color. The colors, from violet, which has the shortest wavelength, to red, which has the longest, are referred to as “visible spectrum“.
Raindrops look more like little spheres than the teardrop shapes we often draw. When light hits one of these small water spheres, the light can change direction. We call this “refraction”.
Each of the different wavelengths is refracted by a slightly different amount. If the light hits the raindrop at the right angle, refraction separates the wavelengths into their different colors. Because a lot of light is refracted by a lot of raindrops, we see these colors as rainbows in the sky. The order of the colors is determined by their wavelength.
When we learn about the rainbow, we are taught that there are seven colors: red, orange, yellow, green, blue, indigo, and violet. But that’s not entirely true.
colors in the rainbow
The different colors blend into each other, and it’s hard to tell where one color ends and another begins. In between are other colors where they mix – like turquoise between blue and green.
Blue and green are next to each other on the color spectrum, which is why we can see turquoise where they blend. However, some colors are mixtures of colors that are not adjacent in the spectrum.
For example, brown is a mixture of red and green. But the red and green bands in the rainbow aren’t adjacent, so we don’t see them blending to become brown. The same is true of many other colors that are blends—if the color bands in the rainbow don’t overlap, they can’t blend.
But there are two colors that we would never see in a rainbow – black and white. Black is the absence of color – it is what we see when there is no light at all.
On the other hand, white is a combination of all colors together. When light is refracted by raindrops, it separates the white light into the visible spectrum, meaning it’s no longer white. Gray is a mixture of black and white, and since we can never see black and white in a rainbow, neither can we see colors that result from mixing.
The next time you see a rainbow, notice how many colors you can see in it—and which colors you can’t see.
James Rawlingshourly paid lecturer in physics, Nottingham Trent University
This article is republished by The conversation under a Creative Commons license. read this original article.
