SciTech

# How Things Work: Rainbows

It?s a beautiful sunny morning after a long night of rain. You look out your window, and there before your eyes is a rainbow! As you stare in awe, you desperately look for its end, to find that lucky pot of gold. But while you stare at nature?s brilliant spectacle, there?s no need to wonder how rain and sun can align to put color in the sky. You?re about to find out.
The fundamental process at work in a rainbow is refraction, the ?bending? of light. Light bends, changing directions when it travels from one medium to another. This happens because light travels at different speeds in different mediums. We see rainbows partly because light travels faster in air than it does in water.
To understand why light bends, imagine you?re roller -skating at constant speed across a concrete parking lot. Your speed depends on the medium you are traveling over ? the paved surface of the lot. If you change the medium you?re traveling through (by roller skating straight onto a grassy area, for example), you?ll slow down. The grass medium offers more resistance, so it takes more energy to move on the roller skates.
When you skate onto the grass at an angle, something else happens. If your right skate hits the grass first, it slows. Your left skate, still on the pavement, is going faster than your right skate and you veer to the right. Moving through different media causes you to change direction.
Just like your skates, a beam of light turns when it enters a glass prism. The light is refracted at the boundary between the air and the glass. (Some of the light actually reflects off the prism surface, but most passes through.) The light bends again when it exits the prism.
If the glass bends the light twice, as in a prism, you can see the separated colors more easily. This color dispersion in a prism occurs because of the refractive index of the glass. Every material has a different refractive index: when white light traveling through the air enters the side of a prism, the difference between the refractive indices of air and glass causes the light to bend. The angle that light bends varies between different wavelengths. As the white light moves through the two faces of the prism, the different colors that make up white light refract at different angles ? and in doing so spread out into a visible spectrum of colors.
In a rainbow, raindrops in the air act like tiny prisms. White sunlight enters the raindrop, bends due to a change in medium, and exits from the raindrop back into air, bending again. In the process, it is broken into the component colors ? red, orange, yellow, green, blue, indigo, and violet ? and we see a rainbow.
If each raindrop gives a rainbow of colors, why do we see bands of color in the sky, as if different areas were dispersing a different color? Your eyes see different colors based on where the raindrop is in the sky. Each color has a different wavelength, and each wavelength is reflected from raindrops at a different angle. Depending on the location of the raindrop, only certain wavelengths are at the proper height to reach your eye. As the raindrop gets lower in the sky, the angle necessary to reach your eye decreases, and you see different bands of color in the sky: a rainbow.
Each rainbow forms a circle of color in the sky; there?s never an end. Therefore, you can never find that lucky pot of gold; sorry to crush your hopes!
And that?s all there is to rainbows. Light and water combine in just the right way to paint a beautiful natural picture. The next time you spot a rainbow, you will see it in a whole new light.