... like I'm 5 years old
A rainbow is a beautiful arc of colors we see in the sky when the sun shines on droplets of moisture. Imagine you're using a water hose in your garden on a sunny day. As the tiny droplets of water scatter the sunlight, they create a mini rainbow.
The sunlight is composed of different colors. When it enters a drop of water, it bends or refracts. As it exits the drop, it bends again, and in the process, it separates into different colors forming a rainbow. The colors of a rainbow always appear in a specific order with red on the outer part and violet on the inner side.
Think of it like this: each water droplet is a tiny prism that takes in white sunlight, and like a skilled artist, it separates the light into its component colors, painting a beautiful arc of colors in the sky.
... like I'm in College
A rainbow is a result of three processes: refraction, reflection, and dispersion. When sunlight strikes a raindrop, it slows down and bends as it moves from air to water. This is refraction. The light then reflects off the inside surface of the drop. As the light exits the drop, it bends again. This double refraction and reflection cause the light to separate into its different colors, a process called dispersion. This is why we see a spectrum of colors.
The colors in a rainbow are always in the same order because of their wavelengths. Red light has the longest wavelength and bends the least, while violet light has the shortest wavelength and bends the most.
Imagine a single white Lego brick as a beam of sunlight. Now, picture this brick being split into seven different colored bricks: red, orange, yellow, green, blue, indigo, and violet. This is what happens when sunlight hits a raindrop.
The prism effect of the raindrop separates the white sunlight into its different constituent colors. When you build an arch with these Lego bricks, placing the red brick on the outer edge and the violet brick on the inner edge, you've just built a Lego rainbow!
Just remember, each water droplet acts like a tiny prism, splitting the white sunlight into different colored bricks, which we then see as a beautiful rainbow in the sky.
... like I'm an expert
From a scientific perspective, a rainbow is a phenomenon of optics that result in a spectrum of light appearing in the sky when the sun shines onto droplets of moisture. It's a complex interplay of light bending (refraction), bouncing (reflection), and splitting (dispersion) inside the raindrops.
Rainbows always appear in the section of sky directly opposite the sun and are centered on the antisolar point. The radius of the rainbow is determined by the refractive index of the water droplets. The primary rainbow forms at an angle of 42 degrees from the antisolar point, while a secondary rainbow, fainter and with colors reversed, forms at an angle of about 51 degrees.