Why Are Rainbows Arched? Physics Reveals the Spectrum Is Circular

After it rains and the Sun comes back out, beautiful colors appear in the sky as a rainbow. The colors sometimes make a perfect arch and look like a curved, short streak in the sky. So, what is the rainbow's actual shape?

How Are Rainbows Formed?

Rainbows are created when sunlight and raindrops form a color spectrum. Light from the Sun appears white, but it is composed of several different wavelengths of light.

Meanwhile, raindrops are commonly considered teardrop-shaped, but they are spherical drops of water suspended in the air. Light starts to bend or refract when it enters a raindrop because water is denser than air. As the light travels, it reaches the back of the raindrop until it bounces off. On its way out of the raindrop, the light refracts once more and separates into its iconic colors. The wavelengths of sunlight refract towards our eyes in slightly different directions, allowing us to see the different colors.

An observer's ability to see a rainbow depends on their position about the Sun. The visible portion of the spectrum appears as the light is reflected and refracted at a certain angle, about 40 degrees from the source. To make this possible, the shadow of the observer's head should be positioned opposite the Sun with no obstructions from clouds. It can be in the middle of the circle even if the entire circle is not visible. This is how we can see the light being refracted and reflected through the raindrops.

The sunlight also needs to emerge from many raindrops at once. We usually observe a rainbow as a flat sky image, but it is more of a mosaic. It only appears flat because the sky has no visual cues to tell us otherwise. As a result, the human eye naturally perceives it as a flat surface.

What is the Rainbow's True Shape?

A rainbow is not just a half circle or an arch but a full circle. The only way to see it in its full circular shape is to observe it from up in the air. This is because the horizon blocks our view of the other half of the rainbow while we are on the ground. This gives us the wrong impression that rainbows are colorful arches that stretch high into the sky and end somewhere in the distance.

Since raindrops are spherical, they reflect light in a cone shape, making rainbows appear circular. Meanwhile, the sunlight reflects most strongly at a 42-degree angle, which can be observed if we hover in the air. But because we are usually on the ground, we see this color spectrum as a curved arc intersecting with the ground.

As a person moves, the rainbow appears, staying in the same spot in the sky relative to the Sun. We can only see it change if we look at it over a long period since the Sun rises or sets across the sky.

Check out more news and information on Rainbow in Science Times.