O Rainbow is a phenomenon of optical nature characterized by the formation of a bright and colorful arc in the sky due to the presence of water droplets suspended in the air. There are many myths surrounding this phenomenon and perhaps the most famous is the presence of riches at the end of the rainbow, but as we will see below, there is no end where gold can be!
How are rainbows formed?
The phenomenon that explains the rainbow is the chromatic light dispersion. When a bundle of polychromatic light (white light) suffer refraction when entering a prism or drop of water, it is separated in the seven colors that compose it. This separation occurs because the refractive index of the middle has an inversely proportional relationship with the wavelength of light. Like each color of light that makes up white light has a characteristic wavelength., the refractive indices of the medium will be different for each color, so white light will be separated and scattered when entering a prism.
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The image above illustrates the occurrence of chromatic scattering of light. Note that blue tones, which have the shortest wavelengths, undergo the greatest change in direction, while red tones, which have the longest wavelengths, suffer the least shift.
iris circle case
A potential observer on the ground will only be able to perceive a rainbow if the direction in which it is located forms an angle of approximately 42° with the direction of light coming from the drops suspended in the air. As all drops have a 42° direction with the observer, the phenomenon will always be perceived in the form of an arc of a circle.
The reality is that every rainbow has a circular shape, but this circle cannot be perceived from the ground because of the curvature of the Earth.
In the video above, a crane operator, who was approximately 500 meters high, filmed a rainbow, and what he saw was a circumference perfect. So every rainbow is actually a rainbow circle!
By Joab Silas
Graduated in Physics
Would you like to reference this text in a school or academic work? Look:
JUNIOR, Joab Silas da Silva. "Rainbow or rainbow circle?"; Brazil School. Available in: https://brasilescola.uol.com.br/fisica/arco-iris-ou-circulo-iris.htm. Accessed on June 27, 2021.
