What Exactly Needs To Happen For A Rainbow To Form?

A rainbow extending over a body of water.

A rainbow is a natural optical phenomenon that can be observed when sunlight travels through water droplets in the air. It’s not something you can just walk through or touch. Rather, it is merely an aftereffect of what happens when light travels from one substance to another. In order to understand how a rainbow is formed, you have to learn a little something about how light behaves, how water causes light to bend, and why only certain conditions produce a rainbow.

A Little Context

Even though sunlight looks white to the naked eye, it is actually a mix of all of the colors in the visible light spectrum. All of these colors move in waves, and the length of these waves is what determines what color we see. Red light waves are longer, while violet light waves are shorter. These waves all mix and move together in sunlight, so you usually don't notice the colors individually. When light moves through air, the waves don't bend much. But when light moves from air into water, each wave bends slightly differently. This is because light bends when it suddenly starts traveling through a different medium, like water, and every color bends at a different speed/angle.

As the sun rays go into the raindrop, the light bends because it's going from the air into the water. The light bounces off the back of the droplet and bends again as it leaves the droplet. Because of the way the light bends at different angles for different colors, the white sunlight gets separated into its individual colors. This is called dispersion, which is what causes the colors to spread out and form the rainbow that you see.

What Has to Happen in Order for a Rainbow to Be Seen?

A number of things must occur simultaneously for a rainbow to be visible. To start with, there must be water droplets suspended in the air. This typically occurs after a rain shower, but it can also occur by waterfalls or fountains. Next, the sun must be out. The sun's light must be at a particular angle to pass through the water droplets and bounce back to you. The sun has to be low in the sky, which usually happens in the early morning or late afternoon. When the sun is high, the angle isn't right, so a rainbow won't form in a way that you can see it from the ground.

Your position matters too. The sun has to be behind you, and the water droplets in front. This way, the light will be able to enter the droplets, reflect around inside them, and then come back towards your eyes. Everyone sees their own rainbow, as the exact droplets that reflect light to you are different from the ones that reflect light to the person standing next to you. That is also why you cannot walk up to a rainbow and touch it. As you move, the angles between you, the sun, and the water droplets shift, so the rainbow will look like it’s just following you wherever you go.

Why Are the Colors in a Specific Order?

The colors of a rainbow always come in the same order. From the outside to the inside, the order is red, orange, yellow, green, blue, indigo, and violet. This is because of how much each color bends. Red bends the least, so it is on the outside of the rainbow. Violet bends the most, so it is on the inside. The other colors are somewhere in between these two depending on how much they bend. By the way, as a side note, a rainbow is actually a circle, but from the ground you usually only see the top half of that circle. You see an arc or a curve instead of the whole circle because the horizon cuts off the rest of the circle. If you’re high up, like on an airplane, you would be, in some instances, be able to see the whole circle.

What Is a Double Rainbow, and How Does It Happen?

A double rainbow over a valley.

At times, you might see two rainbows at the same time. This phenomenon is known as a double rainbow. The main rainbow is brighter and maintains the original order of colors. The second rainbow is not as bright, and the colors have the opposite sequence to the first rainbow. This happens when the light in the water droplet reflects twice instead of once. In the second reflection, the light is slightly weaker, so the second rainbow is not as vivid. The second reflection also changes the angle at which the light leaves the droplet. That is why the second rainbow is higher in the sky, and the colors are reversed.

There is typically also a darker area between the two rainbows. This area is referred to as Alexander's band, and it looks darker due to the fact that less light is scattered in this area of the sky. Much of the reflected light enters the primary rainbow or the secondary rainbow, leaving that middle area less lit up.

Why Don't We See Rainbows All the Time?

Rainbows do not appear every time it rains because the conditions aren’t always perfect. The size of the water droplets matters, as when the droplets are too minute, like in mist or fog, the colors become fuzzy and create a white or colorless rainbow, which is sometimes referred to as a fogbow. Adding on, if the sun is in the wrong place in the sky, the angle will not be right for the light to return to you in the way needed for there to be a rainbow. If there are too many clouds, sunlight cannot pass through to the water droplets, so there is nothing to reflect and bend. Also, if the sun is in front of you instead of behind you, the reflected light goes away from your eyes, and you will not see anything.

In a rainbow, the angle between the light from the sun and the light that’s reflected is always the same. It’s around 42 degrees from the direction opposite the sun. This means that even if everything is set up exactly how it needs to be, a rainbow will only appear if your viewing angle lines up with that specific direction.

Is There an End to a Rainbow?

This is a question that has pestered people for ages: is there really an end to a rainbow? Well, a rainbow doesn’t have an exact location where it ends, and, unfortunately, this means that you will never find that pot of gold at the end of a rainbow either! The rainbow, as mentioned before, is a circle around the point directly across from the sun, but the bottom half is usually invisible to you when standing at sea level.