00:01Why do rainbows have seven colors, and where do they come from?
00:06The sun drenches our planet in different kinds of radiation,
00:10waves of energy that have many different lengths.
00:13Most are invisible.
00:15Like infrared radiation, for instance, which makes you feel warm.
00:20Or UV radiation, which causes sunburn.
00:24The range is wide.
00:26In a narrow window, though, when the waves are between 380 and 780 nanometers long,
00:33we can see them.
00:35That's what we call the visible spectrum.
00:37When all of those wavelengths are mixed together in sunlight, they look white.
00:42But under the right conditions, that white light can be split into its individual colors.
00:47And that's when we see a rainbow.
00:50People often say the rainbow has seven colors,
00:53mostly because that's what Isaac Newton decided during his revolutionary prism experiments.
00:59There are actually no clear boundaries between one color and the next in the visible spectrum, though.
01:06A rainbow is a smooth progression from violet, with the shortest waves, to red, with the longest.
01:12The colors appear when countless water droplets hanging in the air act like tiny prisms.
01:20This is how it works.
01:22Because water is optically denser than air, light slows down when it passes through a droplet.
01:28This alters its path, a process called refraction.
01:31The light then bounces back off the inner surface on the opposite side.
01:36Depending on how long or short they are, the different wavelengths slow by different amounts.
01:42So they spread out and leave the droplet at slightly different angles.
01:46The white light has been split.
01:49But for us to see a rainbow, a few things have to line up perfectly.
01:54First, the air in the direction you're looking has to contain huge numbers of droplets,
02:00which is why you usually see them around the time it rains.
02:04Second, the sun has to be behind you and fairly low in the sky.
02:08And third, the light has to strike the raindrops in front of you at an angle of 42 degrees.
02:17That's why you might also spot rainbows near waterfalls on sunny days because of the droplets in the spray.
02:24So stand in the right place at the right time with the sun shining on moist air
02:30and you may just catch a glimpse of one of nature's most colorful displays.
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