(WHTM) — Sometimes, after it rains and the sun comes back out, you can see some beautiful colors in part of the sky. Sometimes, the colors make a perfect arch, and sometimes it looks like a curved, short streak in the sky. These phenomena are known as rainbows.

But, why rainbows are always curved in an arch shape? First, let’s talk about what makes a rainbow appear in the sky.

To have a rainbow appear, the sun needs to be behind the viewer and rain needs to be falling in front. According to EarthSky.org, when a rainbow is made, the sun is emerging from many raindrops at once. It is not a flat image of the sky, it is more of a mosaic. The reason it appears to be flat is that the sky doesn’t have any visual cues to tell us otherwise, so our eyes naturally see it as being a flat surface.

Also, a rainbow is not just a half circle or an arch. It’s actually a full circle. The only way to see this is from up in the air. This is because the horizon blocks our view of the other half of the rainbow when we are on the ground.

But why are they always in a circle (or half circle)? There is a reason.

Essentially it has to do with geometry and physics. Most rainbows occur when the sun is lower in the sky, usually around sunset or in the mid to late afternoon. When it rains, there are a collection of suspended droplets in the atmosphere that are able to concentrate the sunlight at angles between 40 to 42 degrees, relative to the original path of light from the sun, Physicsclassroom.com explains.

Because the light hits the rain at an angle, the light starts to curve.

Beams of light can bounce any which way, up, down, or sideways. But the only ones you see are the ones that lie on a cone shape, with the angle being between 40 and 42 degrees with your eyes being the tip of that cone.

That is the reason why rainbows are curved. It’s when the sun hits the droplets at just the right angle, that is when a rainbow appears. That’s why a lot of the time after it rains and the sun peeks out, most never get to see one. Either because the sun is too high up, rain isn’t falling hard enough, among other reasons.

For more information about rainbows and a more in-depth look at the physics behind them, click here.