The sun is our major source of light and heat for our planet. It spreads a variety of distinct colours across the sky every day. It is fascinating to see the sky change its hues from light red to bright golden shimmer and then to a pink glow during the sunset. Just by looking at the sunlight, one often thinks that sunlight is yellow and then, it gradually changes to orange. However, upon closer scientific inspection, the facts point to something else entirely - that sunlight is actually white in colour. But on deeper inspection, the white light of the sun is divided further into seven colours of the rainbow. So, let us understand what colour sunlight exactly is and why it is so.
Let us visualize the colours present in sunlight with the help of a simple experiment:
Take a glass prism and set it up in a dark room near the window such that direct sunlight falls on the prism through a small aperture in the window.
Fix a whiteboard to capture the rays passing out of the prism.
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The observation shows that the white board has a spectrum of seven distinct colours, which are the colours of the rainbow, i.e., violet, indigo, blue, green, yellow, orange, and red.
Thus, we can conclude that the white light of the sun rays is actually made of a combination of seven constituent colours.
The sun’s rays are actually white in colour and form a mixture of the seven colours we see in a rainbow, i.e., Violet, Indigo, Blue, Green, Yellow, Orange, and Red, commonly called VIBGYOR. The sun appears to have different colours during the course of a day because of a process called dispersion. Dispersion of light is a phenomenon by which white light splits up into its seven constituent colours, due to the refractive index of the surface of incidence, and the different speeds the different constituent colours have in a medium. The refractive index of a material is defined as a dimensionless number that can describe how fast light travels through a particular material. Different lights have different wavelengths, and hence, different refractive indices in a given material. This causes them to split apart from the original white light, and form a spectrum of the seven colours.
The earth’s atmosphere is made up of various different gases like oxygen, nitrogen, and carbon dioxide. In addition, there are other impurities like dust, smoke particles, and polluting gases like methane and CFCs. When the sun's rays strike the earth’s atmosphere to pass through, they are distorted by the earth’s atmosphere due to the presence of all these materials in the atmosphere. As demonstrated earlier, different colours present in the spectrum have different wavelengths.
The wavelength of a material can be defined as the distance between two successive crests(highs) or two successive troughs(lows) of a wave. So, the longer the wavelength, the lower is the frequency. Thus, blue and violet are scattered more because of their short wavelengths. Conversely, colours of the other end of the spectrum do not get scattered as much, because they have longer wavelengths. Thus, there are different colours of the sun at sunrise and sunset.
When the sun is directly over us during late morning and noon, the sun’s rays are subject to the least amount of interference because the distance traversed by the rays is the least at that time. Therefore, during this period, blue light gets scattered, and the sky appears to be blue, while the sun’s rays appear to be yellow.
During sunrise, the sun is at its farthest, seemingly rising from the horizon. As a result of this, the light rays have to travel a much longer distance through the atmosphere. Therefore, they are obviously subject to more interference, resulting in an increased amount of scattering. Consequently, out of all the colours, the red light is least scattered. The same phenomenon occurs during sunset when the sun is seemingly moving towards the horizon, and as a result, the sunlight colour appears to be of varying shades of red and orange during the dawn and dusk.
1. Why Does A Rainbow Show All Seven Colours Of The White Light Of The Sun?
Rainbows are formed in the sky when the light radiations of the sun come in contact with water droplets in the atmosphere. Rainbows seem to show all the seven colours of the spectrum because the water droplets in the air during/after rain, break the sunlight into the seven colours of the spectrum. This works in the same way as our previously demonstrated experiment of passing sunlight through a prism. The water droplets in the atmosphere act as prisms to achieve a similar effect. When the light rays strike a water droplet, they are refracted at the boundary of air and water, and enter the droplet, causing the white light to be dispersed into the constituent seven colours. Consequently, the brilliant rainbow effect is formed by the light then reflecting inside the droplet and finally emerging out into the air after another refraction at the reflected boundary.
2. How is Light Produced By The Sun?
The sun contains massive quantities of helium and hydrogen, and generates huge amounts of energy at its core, through nuclear fusion, by converting hydrogen to helium. The huge mass of the sun squeezes the hydrogen at its core, in a way where four hydrogen nuclei combine to form a single helium atom. This process is called nuclear fusion, resulting in some mass of the hydrogen atoms being converted into light and heat energy. However, if it wasn’t for the sun’s enormous gravitational force, the star would have exploded due to expansion from the heat generated, which is around 5600℃ on its surface.