How is Rainbow Formed? Why It is Arc Shaped?

The rainbow is one of nature’s most colorful meteorological occurrences that happens due to the qualities and behavior of light and how it interacts with water droplets. Rainbow is one of nature’s most magnificent displays. It is neither a thing nor does it exist in a certain location. A rainbow is an optical illusion or phenomenon that is not controlled by humans and it appears when the sun, air conditions, and the viewer’s position to see it, are all just right. Here we will discuss in detail that how is rainbow formed.

What is rainbow?

A rainbow is a meteorological phenomenon that occurs when light is reflected, refracted, and dispersed in water droplets, resulting in a spectrum of light seen in the sky. It is shaped like a multicolored circular arc. Sunlight-induced rainbows always appear in the sky area directly opposite the Sun. Rainbows can also take the form of entire circles. However, normally the viewer sees simply an arc that is created by the lighted droplets above the ground, centered on a line from the sun to the observer’s eye.

How is Rainbow Formed? Why It is Arc Shaped?

The arc of a primary rainbow is red on the outside and violet on the inside layer. Light is refracted when it enters a droplet of water, reflected internally on the rear of the droplet, then refracted again as it leaves it, resulting in this rainbow. A second arc appears outside the primary arc in a double rainbow, with the sequence of the colors reversed, with red on the inner side of the arc. The light is reflected twice on the inside of the droplet before leaving it, which causes this phenomenon.

How is rainbow made?

A rainbow is formed when sunshine, water and air mix is most apparent on a sunny or wet day. The formation of a rainbow involves physical phenomenon, which includes dispersion, refraction, reflection and total internal reflection of light.

How is Rainbow Formed? Why It is Arc Shaped?

Sunlight and atmospheric factors combine to create a rainbow. As light passes through a water droplet, it slows down and bends as it enters from air to a denser medium: water. The light reflects off the inside the droplet, separating the wavelengths or colors that make up the droplet. A rainbow is created as light escapes the droplet. This occurs when the sun’s rays collide with a raindrop at an angle.

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Rainbows only look semi-circular over level land at sunrise or sunset, when the sun is directly on the horizon; otherwise, only a smaller section of an arc is seen.

Rainbow phenomenon

When the white light from sun hits the water droplets at certain angle then it causes rainbow. The angle at which light strikes the droplet is important because it decides whether the rainbow will appear or not. Usually rainbow is visible at dawn or late afternoon, because at this time sun is low in sky and provides an ideal angle for rainbow. During the interaction of light with water droplet, light is reflected back. The other portion of light enters the drop and slows down, because water is denser than air. The reduction of speed causes light to get bent (refraction). That’s why rainbow is always curved towards the normal line. When this portion of light will escape out the droplet, it will disperse into various wavelengths (dispersion) as during moving from denser to rarer medium, lights bends away from normal. At water to air interface, if angle of incidence will be greater than critical angle then total internal reflection will occur. In other case, there will be no rainbow.

How is Rainbow Formed? Why It is Arc Shaped?

Since water is denser than air, the light flowing through an angle from the air to a raindrop slows and changes direction, a process known as refraction of light. Sunlight is made up of several wavelengths that slow down at varying rates, causing white light to disperse into the blue color with shorter wavelength changing direction slightly more than the longer wavelengths of the red light.

Some of the light that enters the droplet will be internally reflected from the inside edge of the drop and will escape the drop, undergoing refraction of light. The diffused sunlight will be reflected back to the spectator if they are at the right spot. When light from multiple drops reaches the eye of the observer, it appears as a colorful rainbow. Different colors emerge from the droplets at different angles, ranging from red to violet by around two degrees. As seen in the figure below, the red light observed by an observer comes from droplets slightly higher in the atmosphere than the droplets that scatter violet light towards the observer:

How is Rainbow Formed? Why It is Arc Shaped?

Although the human eye is capable of seeing a wide variety of colors, it is still popular to think of a rainbow as consisting of seven colors. The colours in rainbow are red, orange, yellow, green, blue, indigo, and violet (ROYGBIV).

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When rainbow appears?

How is Rainbow Formed? Why It is Arc Shaped?

For the rainbow to appear, water droplets must be floating in the air and clouds must have cleared the sun. That’s why rainbow appears after the rain. Rainbow is formed due to the scattering of sunlight from raindrops into an observer’s eyes. For a rainbow to occur, the sun and raindrops must be in the exact right positions with respect to the observer:

  • The sun must be behind the observer.
  • The sun must be low in the sky, less than 42 degrees above the horizon. The spectator will see more of an arc of a rainbow as the sun lowers in the sky.
  • There must be rain, fog, or any other source of water droplets in front of the spectator.
  • Although the amount of raindrops has no effect on the geometry of a rainbow, mist or fog tends to diffuse the effect more.

Why rainbow is formed in arc shape?

A compete rainbow is actually a full circle but from the ground level, we are just able to see a portion of it. However, complete rainbow can be seen from an airplane, under right conditions.

What causes double rainbows?

How is Rainbow Formed? Why It is Arc Shaped?

We have already discussed that how is rainbow formed. Rarely, it is possible to see another fainter secondary rainbow above the primary rainbow. Since primary rainbow is caused by the reflection of light inside the water droplet, similarly, secondary rainbow is due to the double reflection inside the water droplet. This twice reflected ray exits from drop at an angle of 50 degrees instead of 42. This is the reason due to which secondary rainbow is always above the primary rainbow. Moreover, the order of colors in secondary rainbow is also reversed.

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