Why the sky is blue and other questions


At some point a child will ask “why is the sky blue?” Now days, parents can just pull out a smart phone and use a trusty search engine to find the information. It was not so easy for pre-internet parents. My parents resorted to buying me books. Lots of books. I’m pretty sure this is probably the reason I became a scientist, because I always wanted to know how things work.

So I thought that I would post some quick answers to classic questions asked by children, starting with ‘why is the sky blue?’

‘Why is the sky blue?’

It’s actually a pretty complex answer, and kudos to the parents who have figured out an easy way to explain it to 3 year olds!

The simple answer is that light (sunlight) is actually a type of radiation known as electromagnetic radiation, which consists of colour waves of energy of differing lengths. The length of the waves influences the ability to be detected by the human eye.

When light moves, it encounters obstacles which will stop it from travelling in a straight line. For example, in the atmosphere there are molecules from gases that scatter the light waves. Because blue light waves are shorter, they are scattered in more directions than the other waves and are therefore the predominant light wave that we detect with our eyes.

‘What is a rainbow?’

A rainbow is an optical illusion. It forms in the same way as to how we see a blue sky. Light waves are refracted (bent) when they enter water droplets in the air, so that when the light exits, it separates into the individual colour waves. The order of the colours in the rainbow is due to the respective wavelengths of the colours. Therefore, red has the longest wave and is seen at the top, while violet and indigo have the shortest and are seen on the bottom.

Based on this information, why is the sky not violet/indigo? Violet and indigo light waves definitely have shorter waves than blue, however this means that they are absorbed higher up in the atmosphere. The other answer has to do with how human eyes work.

There are three types of light-sensing structures (called cones) in the cells which respond to different colours. The blue-sensing cones can also detect the small amount of violet that makes it into the lower atmosphere, however the red-sensing cones can also detect the small amounts of violet (and indigo waves) and the net effect between the two cones is that we perceive the sky as mostly as blue.

Why are plants green?

Plants are green because of a pigment called chlorophyll. It strongly absorbs blue light waves but is less able to absorb green light waves and therefore reflects it, making leaves appear green. The energy from the blue light waves is used by chlorophyll to convert carbon dioxide in the air, and water, into sugar (glucose) and oxygen which provide the basic nutrients a plant needs.

So, hopefully this provides some simple answers to some complex questions! Feel free to leave a comment with other questions!








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