How light bends in water droplets
When sunlight enters a water droplet, it slows down and bends, a process called refraction. Inside the droplet, the light bounces off the back surface through reflection. As the light exits the droplet, it bends again. This combination of bending and bouncing separates white sunlight into different colors because each color bends at a slightly different angle.
Why you see different colors
White light is actually made up of all colors mixed together. When light passes through water droplets, each color bends a different amount. Red light bends the least and appears on the outer edge of the rainbow, while violet light bends the most and appears on the inner edge. This separation of colors is called dispersion.
The geometry of rainbows
Rainbows always form in a curved shape because of geometry and physics. Light reflects inside water droplets at an angle of about 42 degrees. This means all the droplets that create the rainbow you see are positioned at this specific angle from the sun. Since these droplets form a circular cone around the point directly opposite the sun, you see a curved arc in the sky.
When and where to spot rainbows
Rainbows appear when three conditions are met: the sun is shining, water droplets are in the air, and you are positioned between the sun and the water. This is why rainbows typically appear in the late afternoon when it rains and the sun is low and behind you. You might see rainbows near waterfalls, fountains, or even in the mist from a garden hose.
Double rainbows and other phenomena
Sometimes you can see a double rainbow, where a fainter second rainbow appears outside the first one with reversed colors. This happens when light reflects twice inside the water droplets instead of once. The secondary rainbow is dimmer because more light is lost with each reflection. 
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