Intermediate — Why Do Rainbows Form After It Rains?

A photo showing a rainbow with its spectrum of seven colors.

This article was written at an intermediate ESL level, so it is most ideal for learners who have a basic understanding of English and are ready to improve their fluency with more complex grammar and vocabulary. For advanced or beginner level articles, visit the “ESL” page.

After a rainstorm, people often look up and feel fascinated when a rainbow appears across the sky. Its bright colors are ROYGBIV: red, orange, yellow, green, blue, indigo, and violet. It feels almost magical. Many cultures see rainbows as symbols of hope, peace, or new beginnings. However, rainbows are not magic. They are natural events created by light, water, and the way our eyes see color. The main scientific idea behind rainbows is refraction, which explains how light bends when it moves through different materials or objects. Understanding refraction helps us understand why rainbows appear after rain and why they always follow the same colorful pattern.

What Is a Rainbow?

A rainbow is an optical phenomenon that happens when sunlight passes through tiny drops of water in the air. These water droplets act like small prisms. When light enters them, it changes direction, separates into different colors, and then reflects back out towards our eyes. A rainbow is not a physical object that exists in one place. Instead, it is something we see because of how light travels and how our eyes receive it. Rainbows usually appear after rain because rain leaves many water droplets floating in the air. When the sun comes out while these droplets are still present, the conditions are perfect for a rainbow to form. The sun must be behind the observer, and the water droplets must be in front. This position allows sunlight to enter the droplets at the correct angle and create a colorful arc in the sky.

What Is Refraction?

Refraction is the bending of light when it moves from one material to another, such as from air to water or from air to glass. Light travels at different speeds depending on the material it is moving through. When light slows down or speeds up, its direction changes. This change in direction is called refraction. Refraction happens all around us. For example, when you place a straw in a glass of water, it may look bent or broken at the surface. This is not because the straw is actually bent. It is because light bends as it moves from water to air. The same idea applies to rainbows. When sunlight enters a water droplet, refraction changes the direction of the light and starts the process that creates a rainbow.

How Does a Rainbow Form?

The formation of a rainbow happens in several clear steps, and refraction plays a key role in almost all of them. First, sunlight enters a water droplet in the air. As the light moves from air to water, it slows down and bends. This bending is refraction. At this moment, the white sunlight begins to separate into the different colors that make it up. Each color bends slightly differently. Red light bends the least, while violet light bends the most. Next, the light reflects off the inside surface of the water droplet. This reflection sends the light back toward the direction it came from. The reflection helps concentrate the colors and sends them back out of the droplet. Lastly, as the light leaves the water droplet and returns to the air, it bends again. This second refraction increases the separation of colors even more. By the time the light reaches our eyes, the colors are distinctly separated, forming the rainbow we see in the sky.

Each color in a rainbow appears at a slightly different angle. Because of this, red usually appears on the outside of the rainbow, while violet appears on the inside. Our eyes receive light from millions of droplets at the same time, and together, they create the full arc of the rainbow.

Why Are Rainbows Curved?

Rainbows often appear as curved arcs rather than straight lines. This shape is related to the angle at which light enters and leaves water droplets. The light that forms a rainbow reaches our eyes at a specific angle, usually about 42 degrees for red light and slightly less for violet light. Because water droplets are spread out evenly in the air, the light that reaches our eyes forms a circular shape. From the ground, we usually see only part of this circle, which looks like an arc. From airplanes or high places, people can sometimes see a full circular rainbow. The curved shape reminds us that a rainbow depends on the position of the observer. If you move, the rainbow will appear to move with you. This happens because you are seeing light from different water droplets as you change position.

Why Do Rainbows Have Seven Colors?

Rainbows are often described as having seven colors, but in reality, they contain a continuous range of colors. The idea of seven colors comes from how humans categorize color. When sunlight separates, it spreads into a spectrum of colors. Our eyes and brains group these colors into seven categories, which includes red, orange, yellow, green, blue, indigo, and violet. Refraction causes this separation because different colors of light have different wavelengths. Shorter wavelengths, like violet, bend more during refraction, while longer wavelengths, like red, bend less. This difference is why the colors separate out instead of staying mixed together as white light.

Why Don’t We See Rainbows All the Time?

Even though sunlight and water are common, rainbows require very specific conditions. The sun must be low in the sky, usually in the early morning or late afternoon. If the sun is too high, the angles will not work out, and a rainbow will not form. There also needs to be enough water droplets in the air. Light rain, mist, or spray from waterfalls can create rainbows. Without these droplets, there is nothing for the light to refract through. Clouds alone are not enough. Finally, the observer must be in the right position. As mentioned before, the sun must be behind you, and the water droplets must be in front of you. If these conditions are not met, refraction and reflection will not create a visible rainbow.

Other Types of Rainbows

Not all rainbows look the same. Sometimes, you may see a double rainbow. A double rainbow forms when light reflects twice inside a water droplet instead of once. The second rainbow appears outside the first and has reversed colors. It is usually fainter because more light is lost during the extra reflection. There are also fogbows, which form in fog instead of rain. Fogbows appear white or very pale because the water droplets are much smaller. Moonbows are rainbows created by moonlight instead of sunlight. They are rare and often difficult to see clearly. Regardless, all rainbows depend on refraction, reflection, and the relationships between light and water.

Why Is Understanding Refraction Important?

Learning about refraction helps us understand more than just rainbows. Refraction is used in many areas of science and technology. Glasses and contact lenses use refraction to help people see clearly. Cameras, microscopes, and telescopes rely on refraction to focus light. Even fiber optic cables, which carry internet data, use principles related to how light bends and travels. By studying rainbows, scientists can understand light better. This knowledge has helped improve technology, medicine, and communication. Rainbows are a simple and beautiful example of how science explains everyday experiences.

Why Should We Care?

Rainbows remind us that science is not separate from daily life. Something as common as rain and sunlight can create a beautiful event through simple interactions. Understanding refraction allows us to appreciate rainbows not only for their beauty but also for what they teach us about the natural world.

Glossary

Here are the definitions for some of the important terms discussed in this article:

• Rainbow: A colorful arc in the sky caused by light passing through water droplets.

• Refraction: The bending of light when it moves from one material to another.

• Reflection: When light bounces off a surface instead of passing through it.

• Spectrum: A range of colors produced when light is separated.

• Wavelength: The distance between waves of light, which affects color.

• Angle: The direction or position at which something moves or is seen.

• Prism: A transparent object that separates light into different colors.

• Observer: A person who is watching or looking at something.