Sunlight is consisting of multiple wavelengths of light. It includes wavelengths of light which is not visible to us, such as infrared and ultraviolet light, as well as the visible spectrum. Reflection, refraction and dispersion of sunlight inside water droplets in the atmosphere, causes the formation of rainbow which appears as a spectrum of light in the sky. Rainbow appears as a circular arc with various colors and it always seen in the section of sky which is directly opposite the sun.
A rainbow is not a physical object and it cannot be approached. It does not exist at one particular location. It is a continuous spectrum of colors. It can be caused by many forms of water in the air like rain, mist, spray, and airborne dew etc. The colors of the rainbow cannot be observed unless the light is refracted inside a water droplet. A prism can also be used to refract light in a laboratory and it will reveal the components of white light with various wavelengths. Rainbows consist of a continuous spectrum of colors. We will not be able to see any distinct band of colors, but only a smooth gradation of intensity is observable. We can see seven colors: red, orange, yellow, green, blue, indigo and violet and it can be remembered by the mnemonic, Richard Of York Gave Battle In Vain (ROYGBIV).
There are mainly two types of rainbow; Primary and Double. In a primary rainbow, the red color is on the outer and violet appearing on the inner portion of the arc. This type is rainbow is caused by the light which is refracted when entering a droplet and then undergoes total internal reflection inside on the back of the droplet and refracted again while exiting. In a double rainbow two arcs of colors appear. A second arc is formed outside the primary arc. The secondary arc has reverse order of colors, with red on the inner side of the arc. This is formed because the light gets reflected twice on the inside of the droplet before exiting.
The process of formation rainbow is interesting. When sunlight hits on the surface a raindrop, part of the light is reflected and the remaining part enters to the interior of the raindrop. When light goes from air to droplet, it is refracted at the interface between air and water. The light which enters the droplet, moves forward and hit the other side of the raindrop. Some part of that light is reflected off the back and remaining passes through the interface. Internally reflected light again reaches the surface, and again it is internally reflected and refracted as it exits the drop. The overall effect is that part of the incoming light undergoes total internal reflection over the range of 0° to 42°, with the most intense light having re flection of 42°. Due to different reflection angle components of lights split depending upon its wavelength and color. This effect is called dispersion.
Light with shorter wavelength (Blue) is refracted at a greater angle compared to the Light with larger wavelength (red). As a result of the reflection of light rays from back of the droplet, the blue colored light exit from the droplet at a smaller angle to the original incident sun light ray than the red light. Because of this angle, red on the outside of the arc of the primary rainbow, and blue is seen on the inside. The result of this is to give different colors to different parts of the rainbow.
The raindrop acts like a prism, and when sunlight passes through it, the wavelengths in white light are refracted and undergo dispersion to reveal the colors of the rainbow. These colors we see always go from red, which is least refracted, through orange, yellow, green, blue, indigo and violet. The blue, indigo and violet wavelengths are refracted the most as sunlight passes through raindrops.