Rainbow

Rainbow

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc. The colours of the rainbow are red, orange, yellow, green, blue, indigo and violet.

In a primary rainbow, the arc of a rainbow shows red on the outer (or upper) part of the arc, and violet on the inner section. This rainbow is caused by light being refracted then reflected once in droplets of water.

The light is first refracted entering the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of 40–42°. The angle is independent of the size of the drop, but does depend on its refractive index.

The speed of light in an optical medium is different for different wavelengths. When a beam of white light passes from air into an optically denser medium, all seven constituent coloured lights travel slower and undergo refraction. As a result of difference in speed, each wavelength slows down at different rates and are refracted at different angles upon entering and exiting the optical medium. This is how the dispersion of white light is achieved.

The amount by which light is refracted depends upon its wavelength, and hence its colour. This effect is called dispersion. Blue light (shorter wavelength) is refracted at a greater angle than red light, but due to the reflection of light rays from the back of the droplet, the blue light emerges from the droplet at a smaller angle to the original incident white light ray than the red light. Due to this angle, blue is seen on the inside of the arc of the primary rainbow, and red on the outside.

Double Rainbow

Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50–53°. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky.

Reflections:

I feel that the topic on rainbow is very interesting. I learnt many things from this like the different types of rainbow, formation of the rainbow and the dispersion of light. I never heard or seen a double rainbow before. Although a dim secondary rainbow is often present outside the primary rainbow, most people will not notice it because they are not actively looking for it and also because it is fainter than the primary rainbow as more light escapes from two reflections. In the future, when I see a rainbow in the sky, I would definitely try to spot for the secondary rainbow.