A rainbow displays red, orange, yellow, green, blue, indigo, and violet because white sunlight is made up of different colors that bend and separate when passing through water droplets in the air. Each color bends at a slightly different angle, causing them to spread out in order of wavelength.
How Light Separates Into Colors
White sunlight looks colorless to our eyes, but it is actually made up of many different colors mixed together. Each color has a different wavelength, which is the distance between one wave peak and the next. When sunlight enters a raindrop, it slows down and bends. This bending process is called refraction. Because each color has a different wavelength, each color bends at a slightly different angle. This causes the colors to separate from each other.
The Role of Water Droplets
Water droplets in the air act like tiny prisms. When light enters a raindrop, it bends, reflects off the back of the droplet, and then bends again as it exits. This double bending and internal reflection causes the colors to spread out even more. Millions of droplets work together to create the rainbow we see. Each droplet reflects light at a specific angle, which is why rainbows always appear at roughly the same angle from the sun.
Why Colors Appear in a Specific Order
Red light has the longest wavelength and bends the least, so it appears on the outer edge of the rainbow. Violet light has the shortest wavelength and bends the most, so it appears on the inner edge. Orange, yellow, green, blue, and indigo appear in between in order of their wavelengths. This is why the color order in a rainbow is always the same: red on the outside and violet on the inside.
Conditions for Rainbow Formation
Rainbows only form when specific conditions are met. You need sunlight, water droplets in the air, and the correct angle between the sun, the water, and your eyes. This is why rainbows typically appear when it is raining while the sun is shining. The observer must stand with the sun behind them and the rain in front of them. The sun must be at an angle of less than 42 degrees above the horizon. 
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