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Rainbows have forever been considered symbols of awe and wonder, hope, and natural beauty. However, their sudden appearance post-showering often brings a feeling of magic and awe. Behind this natural spectacle lies a very intriguing and real scientific explanation. Ever wondered how are rainbows formed, or the deeper science behind two big, colorful arcs formed in the sky? Interestingly, many are curious about what causes this amazing phenomenon and how all the colors blend together in absolute harmony.
Before explaining the process, you should know exactly what a rainbow is. A rainbow is not a physicality you could touch or approach. On the contrary, it is an optical illusion caused by the interaction of sunlight with water droplets in the atmosphere. Under suitable circumstances, sunlight hitting the raindrops splits into a spectrum of colors that form circles in the sky.
The Earth's surface obstructs the bottom of the circle, making the rainbow look like an arc. A rainbow can be formed whenever water droplets are suspended in the air and sunlight shines from behind the observer at a low angle.
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Now, joining the matter of the science of rainbow formation. Light travels in waves and contains many different wavelengths, a wavelength associated with a particular color. When sunlight, which appears white to our eyes, passes through water droplets, it undergoes a series of changes: refraction, reflection, and dispersion.
First, one should understand refraction. The entrance of sun rays into a water drop causes refraction. In other words, when light passes from air (a less dense medium) into water (a denser medium), it is slowed down and bent. This bending at the entrance of light into the water drop is the first stage in the formation of the rainbow.
Inside, the light will reflect from the droplet's surface. This internal reflection lets the light bounce back towards the front of the droplet. Then, the light is once again bent as it leaves the droplet is refraction. During this exit phase, the dispersion of light into its various colors occurs.
The varying wavelengths bend at different angles. Red light bends least, and violet bends the most, which causes their separation into colored bands and thus gives a rainbow its color order. The science behind a rainbow is a beautiful mixture of physics and nature. It shows that the bending and bouncing of light lead to an outstanding visual spectacle.
If we can understand sunlight, water droplets, and specific viewing angles, then this can give us the correct answer on how rainbows are formed. Rainbows appear whenever sunlight travels through rain or mist in the air. The conditions necessary for rainbow formation are the sun's low in the sky (early morning or late afternoon) and moisture in the air, usually from rain or mist.
Sunlight enters a raindrop and is refracted; the light is then reflected off the inside surface of the droplet and refracted again as it exits. The second refraction changes the direction of the light and breaks it into various colors. Every droplet in the sky imparts just one color to the rainbow because only certain rays emanating from certain droplets enter your eyes at the right angle. Hence, each person experiences their rainbow differently from a friend standing beside them.
Each drop of water refracts light; with this happening billions of times together, every drop finishes its contribution of a color to complete the entire arc. This partly brings out the illusory and personal nature of every rainbow.
The shape of a rainbow is one of the most curious aspects of its formation. Why is the rainbow always a curved arc. Due to the constant angle of 42°, light is refracted, reflected, and dispersed inside water droplets to reach the human eye.
This specific angle means that light must come from a particular band of the sky. Because the process occurs in a three-dimensional space, the light exiting the raindrops at this angle creates a circle. However, from the ground, we usually see only the upper half of this circle—the arc. If you were in an airplane or on a mountaintop, with the sun behind you and rain below, you might see a complete circular rainbow. This curved shape further highlights the precision involved in the science behind rainbows and the importance of the observer's position in seeing the rainbow.
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The cause of rainbow formation goes into the physics of light and the corresponding atmospheric conditions that accommodate it. The single and most essential reason is the interplay between sunlight and spherical water droplets suspended in the atmosphere. If either of the two is missing, the rainbow will not appear.
An important factor for the dimming of the rainbow is that the Sun must be low in the sky less than 42 degrees above the horizon. If it is higher, the angle of refraction and reflection won't match with your eyes, and you just won't see a rainbow. This is why most rainbows are seen during dawn or evening hours and very rarely at noon.
Moreover, you must keep the Sun at your back, looking into the shade where the rain occurs. That is the setting, and that is where the elements come together to produce your rarer-than-hen's-tooth occasion of nature's enchantment.
On a rainbow, seven color hues, which act as a separation of white light, can be seen: Red, Orange, Yellow, Green, Blue, Indigo, and Violet, hence the common ROYGBIV mnemonic. These colors will always go in that order because that is how light bends in dispersion.
Red light, which has the longest wavelength, bends the least and appears on the outer edge of the arc. Violet, with the shortest wavelength, turns the most and appears on the inner edge. The smooth transition between these colors occurs because the wavelengths of visible light blend seamlessly. This separation and order are consistent, regardless of where or when the rainbow appears, reinforcing the systematic nature of the science behind a rainbow.
Interestingly, some people with acute vision or special photographic equipment can see additional bands between the primary colors, subtle transitions that further demonstrate the complexity of rainbow formation.
If there is ever a second, dimmer rainbow that appears outside the brighter one, you have caught sight of a double rainbow. It appears when sunlight reflects twice inside the raindrop instead of just once. The second reflection makes the light leave the raindrop at a different angle, usually at around 50 or 53 degrees, thereby producing a second arc above the primary rainbow.
The secondary rainbow has some fascinating features. The colors are reversed: red appears on the inner edge while violet is on the outer edge. With this inversion and spread, double rainbows become truly eye-catching. The region between the two arcs looks darker due to the presence of Alexander's Band, named after Alexander of Aphrodisias, who first described the obstruction in 200 AD.
Other varieties of rainbows include fogbows, lunar rainbows (or moonbows), and circumhorizontal arcs. All arise from the same principles but differ in scale, degree of coloration, and the nature of the droplets involved.
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The science behind rainbows is interesting and wonderful proof that laws of nature govern even the most magical sights. Every time a rainbow appears, one hundred percent natural forces align one time only. The formation of a rainbow is a perfect combination of refracted light, internal reflection, and sunlight dispersion throughout billions of water droplets.
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