What is a rainbow?

When we see sunlight streaming through the windows, it appears colorless. Although we cannot see it, this “white” light is actually made of many different colors of light, and each of these colors has a different wavelength.

In 1666, the famous scientist Isaac Newton discovered that if sunlight passes through a triangular piece of glass called a prism, then the white light will split into a group of colors.This color group consists of red, orange, yellow, green, blue, indigo and violet. These are the colors of the rainbow in order.

A rainbow, a series of concentric colored arcs that can be seen when light falls from a distant source – the most common sun – onto a group of water droplets – such as in rain, drizzle, or fog. A rainbow is observed in the opposite direction to the sun.

The colored rays of the rainbow are caused by the refraction and internal reflection of the rays of light entering the raindrop, with each color being bent at a slightly different angle. Thus, the compound colors of the incident light will be separated at the exit of the fall.

The brightest and most common rainbow is the so-called primary arc, which results from the light that emits from the droplet after one internal reflection.

Although the light rays may depreciate in more than one direction, a high intensity of the rays appears at the slightest angle of deviation from the direction of the incoming rays.Thus the observer sees the highest intensity when looking at the rays with the lowest deviation, which form a cone with the head in the eye of the observer and as the axis passes through the sun.

The light emitted by raindrops after one internal reflection has a minimum deflection of about 138 degrees, so the maximum intensity in directions forms a cone with an angular radius of about 42 degrees, with arcs (inside to outside) of violet, indigo, blue, green, yellow and orange. And red.

Red has the longest wavelength of visible light, around 650 nanometers. It usually appears on the outside of the rainbow. Violet has the shortest wavelength (about 400 nm) and usually appears on the inner arc of a rainbow.

Occasionally, a secondary arc may be observed, which is much less intense than the primary arc and reflects its color sequence. The secondary rainbow has an angular radius of about 50 ° and is therefore seen outside the primary arc.

This arc is produced by light that has undergone two internal reflections inside the water droplet. High-order rainbows, caused by three or more internal reflections, are very weak and thus rarely noticed.

Occasionally, faded-colored rings appear inside the primary arc. These are called trailing rainbows; Its origin is due to the interference effects of light rays emerging from the water droplet after one internal reflection.

References :



“Why Do Rainbows Appear?”,

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