What makes a rainbow?
A beautiful rainbow forms where sunlight meets the rain.
Light is made up of many different colors, all of which have a different wavelength. Normally when we see light from the sun it appears colorless. So how is it that colorless light gives us colors? What makes a rainbow?
The Spectrum of Colors
Sunlight is made up of the whole range of colors that the eye can detect. The range of sunlight colors when combined looks white to the eye. Light of different colors is refracted, or bent, by different amounts when it passes from one transferring medium, like air, into another medium, like water or glass. If you take a prism, you see a spectrum of colors. The band of colors that appears is red, orange, yellow, green, blue, indigo, and violet.
The Location of the Sun
When you see a rainbow you first have to keep in mind the position of the sun relative to you, the observer of the rainbow. This physical process is what gives rise to a rainbow. When you see a rainbow the sun is always behind you; when you face a rainbow, the center of the circular arc of the rainbow is in the direction opposite to that of the sun.
The Location of the Rain
If the sun is behind you, the rain is in the direction of the rainbow in front of you. To create the rainbow, you need raindrops.
Because a typical raindrop is spherical, its effect on sunlight along the axis is symmetrical along the center of the drop. This symmetry creates a focusing effect for each drop such that whenever we view a raindrop along the line of sight defined by the rainbow ray, we will see a bright spot of reflected, refracted, sunlight.
What causes the bow? Sunlight and raindrops act upon each other. The round raindrops and the way that the sun's rays hit the droplets cause the colors of the light to spread out in an arc.
Light Interacts with the Raindrop
Many parallel sunlight rays enter the drop over its entire spectrum, so you don't get a single spectrum color leaving the drop. Instead you get many colors overlapping each other to various degrees and each one corresponds to a different point of entry of a ray of light to the drop. So how do you get a nice rainbow from this spectacle of spectrum colors?
The 40 Percent Solution
Sunlight enters the raindrop, then exits the raindrop. If you look at the exit angles of the sunlight as determined from where they enter the drop, an interesting pattern emerges. Sunlight rays aimed exactly at the center of the drop will be reflected straight back at least partially.
But if the ray is off-center, the rays emerge at an angle and as you move the entry point further off-center, the angle of deviation increases. But this only goes so far. This rate of increase slows down, until, with rays aimed near the edge of the drop, the angle of deviation reaches a maximum of about 40 degrees, where it starts to decline once again for rays even further off-center.
The upshot is that when all of the sunlight enters the raindrop at different places, an overwhelming number of rays exit the raindrop at an angle of about 40 degrees, so evenly-illuminated raindrops prefer to reflect light back at about 40 degrees and this angle does not vary much with wavelength, although it tilts more for red and less for blue.
What Makes a Rainbow?
When it rains, the air is filled with raindrops. These raindrops act like a prism. If sunlight passes through the raindrops at the proper angle it is split into its spectrum, which is made up of the colors of the rainbow. The interaction of the round raindrops with the sunlight forms the arc. Sunlight and raindrops acting upon each other in the right combination is what makes a rainbow.