Have you ever wondered why a rainbow always appears as an arc?

In short, it all comes down to the refraction, reflection, and dispersion of light: if you've attended even a couple of physics lessons in school, these terms should be familiar to you. However, let's dive a bit deeper into this complex question.

The appearance of a rainbow relies on just two factors — water droplets and sunlight. Under these conditions, each water droplet acts as a prism; when a ray of sunlight hits a droplet, it can either pass through (but then we wouldn't see it) or refract, reflect off the back of the droplet, and refract again as it exits, but at a different angle. This process initiates dispersion: white light splits into individual colors, from red to violet, each with its own wavelength. Waves of different lengths refract at their own angles, causing the light to break down into the spectrum that we perceive as a rainbow.

As light refracts and reflects at specific angles, there exists a range of angles at which certain colors become visible. For instance, red emerges at an angle of about 42 degrees, while violet appears at an angle of around 40 degrees. To see a rainbow, one must be positioned between the light source (typically the Sun) and the water droplets in such a way that the angle between the observer's line of sight and the incoming light is at those very 40-42 degrees.

In reality, rainbows are circular. However, we usually only see the upper part of them. When you are in an airplane or at the top of a tall mountain, you have the chance to observe a complete circle, but on the ground, only an arc is visible, as the lower part is typically hidden behind the horizon.

And that's it! The mystery of the colorful semicircle is unveiled. Now you can appreciate the beauty of a rainbow, knowing that the stunning spectacle is governed by the laws of physics.