Why is the sky blue?



Have you ever looked up and wondered why is the sky blue? There are so many other colors, right? The sunlight looks white so shouldn’t the sky be white? Well, you aren’t alone. Irish Physicist John Tyndall wondered the same thing almost 160 years ago. The good thing is that he found the answer. So now you can know the reason too. Good thing you weren’t born 160 years ago, huh?

When Tyndall had this question instead of just googling it he decided to find the answer himself. So he devised an experiment. He took a glass tube that was like the sky. He filled it with smoke and put a white light at one end. He thus simulated what happened above. When he observed this setup it appeared blue just like the actual sky.

From his experiments, Tyndall deduced that the blue sky was because Blue light was scattered by dust particles more than the other colors. However, he couldn't explain why blue light was scattered more.

It was ten years later that British scientist, Lord Rayleigh was able to explain Tyndall’s deduction. White light is made up of light of seven different colors,( Violet, Indigo, Blue, Green, Yellow, Orange, Red). The color of light is determined based on a property of light called the wavelength. To explain this phenomenon light is considered as a wave. It is like the ripples in water when a stone is thrown in it. The distance between two such ripples is called a wavelength.

Lord Rayleigh found that the scattering of light by particles that are of the order of the wavelength of light depended on the wavelength of light. Light with a shorter wavelength(Violet, Blue) was scattered more than light with a longer wavelength(Orange, Red).

So now another question arises. Violet light has a shorter wavelength than blue light, so Why isn't the sky violet? There are two reasons for this. One of the reasons is that the sunlight that hits our atmosphere isn't an equal mix of all colors. It is a mix of all colors with Red and orange dominating, and with more blue than violet. The second reason lies in our eyes. Our eyes are more sensitive to blue light than violet light. If there is a blue spot and a violet spot both measured by experiments to be of equal brightness. our eyes will perceive the blue spot to be brighter. These are the reasons as to why the sky isn't violet.



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