The Greek philosopher Aristotle is credited with "discovering" the rainbow back in 350 B.C. However, it wasn't until around 1665 A.D. that Isaac Newton first planted the seeds of what later became a scientific explanation as to why rainbows form the way they do. The seven colors in the rainbow – red, orange, yellow, green, blue, indigo and violet – always appear in the same order.
What Causes Rainbow Colors
While a rainbow may look like it is in a fixed spot in the sky, it doesn't. Everyone sees a rainbow in a different spot, depending on their position on the ground and the position of the sun (or other light source). Another cool fact about rainbows is that no two people ever see exactly the same thing. What you see varies depending on how the light is bent and reflected back to you. Someone who looks like they are standing right underneath a rainbow will actually be looking at it and seeing it from a distance.
The beautiful colors you see when a rainbow appears are caused by light being split by a prism into different wavelengths, which creates a color spectrum. A rainbow usually appears after rain, because when sunlight (visible white light, which is actually a mixture of all visible colors) passes through water droplets, it bends and split into the colors of the rainbow. The water droplets are usually rain drops, but they could also come from waterfall spray, a fountain, mist, dew or fog. Rainbows are made up of seven colors because the water droplets break white sunlight into the seven main colors of the visible-light spectrum.
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However, if you inspect a rainbow more closely you can see that there are more than seven individual colors. A rainbow is not a pure spectrum, but made up of several colors that have overlapped and mixed together. There are simply too many colors for the human eye to distinguish them all.
When the colors of a rainbow overlap at their edges, they produce a sheen of "white" light, which makes the inner part of a rainbow much brighter than its outside.
Some scientists think indigo is too close to blue to be truly distinguishable, which would give a rainbow only six distinguishable colors.
How Rainbows Are Formed
A rainbow forms when sunlight is scattered from raindrops into your eyes. Certain meteorological conditions must apply in order for you to see a rainbow. For starters, the sun must be behind you, low in the sky and at an angle of less than 42 degrees above the horizon. The lower in the sky the sun is, the greater the arc of the rainbow is. Additionally, there must be rain, fog or another source of water droplets in front of you.
When light hits a water droplet at an angle, it slows down and changes direction. This is known as refraction and happens because water is denser than air. When light goes into the water at a certain angle, some of that light is reflected from within the water and then leaves the droplet. As it moves back into the air, the refraction process happens again.
When you see a rainbow, you are actually looking at light refracted and reflected from different raindrops, some viewed at an angle of 42 degrees (red light), some at an angle of 40 degrees (blue light) and some in between. The angle of deviation is different for red light and blue light because blue light is bent (refracted) more than red light.
The best time to see a rainbow is just after a rainstorm has ended, when there is still water vapor in the air. You won't see a rainbow during the rainstorm because clouds block most of the light. If you wait too long after the rainstorm, the water vapor in the air will all have evaporated. You won't see a rainbow after winter weather like snowfall because water droplets freeze into ice particles that scatter light in different ways.
If you are standing in the right place, you'll see the dispersed sunlight reflected back toward you. The colorful rainbow appears when light is scattered by many water droplets, as different colors exit the droplets at varying angles.
A rainbow has a semicircular shape because water droplets do not fall in a flat sheet, but at various distances and speeds. However, in most cases a rainbow appears as part of an arch, semi-circle or "U" shape. You only see a semicircular rainbow over level ground at sunrise or sunset when the sun is right on the horizon. Otherwise, you see a smaller part of the rainbow's arc. Normally, you can only see no more than one semi-circle of a rainbow as the other semi-circle is hidden below the horizon. However if you are in a higher position than the water droplets, such as on top of a tall building, it is possible to see a full circle rainbow. The center of a rainbow is directly opposite the position of the sun in the sky, which means you can see more of a rainbow as the sun approaches the horizon.
If you're lucky, you might be able to see a double rainbow (a secondary rainbow above the main one). The colors are in reverse order in the secondary rainbow, and it is fainter than the primary rainbow because more light escapes from two reflections than from one. The secondary rainbow is dispersed over a wider area of the sky, making it almost twice as wide as the primary rainbow.
The primary and secondary rainbows in a double rainbow often have a dark band in between them. This is called Alexander's band, after Alexander of Aphrodisias, who first described the band in 200 A.D. The darkness of the band is due to the fact that none of the sunlight is scattered toward you by the water droplets between the deviation angles of the primary and secondary rainbows.
Sequence for Rainbow Colors
The sequence for rainbow colors is always the same – red, orange, yellow, green, blue, indigo and violent – because different colors of light have different wavelengths. Red always appears first, forming the rainbow's outer arc, because it has the longest wavelength of around 650 nanometers. Because light bends at 42 degrees to produce this color, you can almost always see it in a rainbow, even if the other colors in the rainbow are not as obvious.
The red light you see in a rainbow comes from drops slightly higher in the atmosphere than the drops that scatter violet light. Shorter wavelengths go through a slightly increased change of direction, meaning violet always appears last on the rainbow's inner outline because it has the shortest wavelength of about 400 nanometers.
Isaac Newton's Color Theory
Around 1665, Isaac Newton passed white light through a prism and watched the light form a rainbow made up of seven different colors. Newton believed that the colors were analogous to the notes of the musical scale beginning with D and containing no sharps or flats. Two of the colors – orange and indigo – corresponded to half steps in the scale. Although Newton's musical analogy was later disproved (when scientists discovered that musical frequencies and visible light wavelengths are not equal), his color theory showed that white light is a mixture of different-colored lights and helped future generations better understand the nature of light.
Other Types of Rainbows
Instead of a rainbow, you might see a fogbow, a moonbow or a red rainbow.
A fogbow is similar to a traditional rainbow, but it forms when sunlight interacts with water droplets contained in fog, mist or cloud, rather than raindrops. The water droplets in a fogbow are between 10 and 1,000 times smaller than raindrops and almost always less than 0.1 mm in diameter. A fogbow may be known as a white rainbow because, unlike a traditional rainbow with seven distinct colors, it is almost absent of color. This is because the water droplets are so tiny. While light still reflects from the water droplet back toward you, the process of diffraction of the light by the droplet has a dominant effect. Diffraction broadens the reflected beam of light, blurring the colors and giving a white or very faintly colored appearance.
A moonbow is sometimes called a lunar rainbow. This occurs when light is refracted from the moon, via drops of water in the air.
Moonbows are very rarely seen because they are so faint. The amount of light produced by even the brightest full moon is much less than the amount of light produced by the sun. Additionally, a full moon does not produce enough light to stimulate cone color receptors in the human eye. However the colors are still there, and can be picked up by long-exposure photography. Moonbows are more common in certain parts of the world, such as Hawaii.
To see a moonbow, the moon must be close to or at its fullest phase and at an angle of less than 42 degrees in the sky. Also, the night sky needs to be very dark, and there must be rain falling opposite the moon or another source of water droplets, like a waterfall.
To see a moonbow, the moon must be behind you. The optimum moonbow-spotting time is a couple of hours after sunset or before sunrise.
If you happen to see a red rainbow at sunrise or sunset, you've spotted a monochrome rainbow. At these times of day, sunlight travels a greater distance in the atmosphere, distributing shorter blue and violet wavelengths. The blue and violet wavelengths cannot be seen by the human eye, so the rainbow appears to be entirely red.
Remembering Rainbow Colors
An easy way to remember the colors of the rainbow in the correct order is with a mnemonic, a phrase that takes the first letter of each color and makes up a new word. When put together, the words form a phrase that is easy to remember. A common mnemonic for the rainbow colors is Richard Of York Gave Battle In Vain, but it's easy to create one that appeals to you.
Another easy way to remember the sequence of the colors of a rainbow is the name "Roy G. Biv."
Make Your Own Rainbow
All you need to make your very own rainbow is the sun and a water hose. Stand with your back against the sun so you are facing away from it. Spray the water hose up into the air to see a mini rainbow. Move the hose up or down if you need to. This works best on very sunny days.
Another way to create a rainbow is to hold a glass prism up to a window to let the light shine through it.