The Nature of Colour – Why Plants are Green and the Sky is Blue

Those of us able to see the full range of colours often take colour for granted. We consider it to be an implicit characteristic of an object – If you have a red ball, it is red just as obviously as it is a ball – rather than something affected by its environment. To actually understand the nature of colour you need to take a dive into the deep, muddy pool of quantum physics. I’m only just starting my physics degree, so this is just going to give you a basic run down.

Light is an interesting form of energy. It consists of massless particles called ‘photons’ that travel at the unimaginably fast speed of 300 000 000 metres per second (and yet it still takes eight minutes and twenty seconds for sunlight to reach the Earth). Photons act as both a wave and a particle which results in some very interesting behaviour. To describe colour we look to its wave-like properties.

That squiggle does everything from cook your food to make your t-shirt glow at night clubs

That squiggle does everything from cook your food to make your t-shirt glow at night clubs

The above picture is the visible light spectrum; the range of colours that compose visible light. All of these colours together make white light, while an absence of all of them leaves black. Below the spectrum is the relative wavelengths of the different colours of light. This difference in wavelength means that when different colours of light come into contact with an object they will behave differently.

I first started learning about light in my high school physics class. The thing that fascinated me the most is that the colour we see when we look at a physical object is the ‘reject colour’: the one that is reflected or transmitted rather than absorbed by the object. So the reason that ball was red wasn’t because its quantum nature designates it to be red, but because it better absorbs a certain colour of light and leaves behind a combination of colours that result in red. So if you were to view that red ball under a green light, it would appear black because it is absorbing (most of) all the available wavelengths of visible light .

Let's just say that playing snooker under coloured lights would be tricky

Probably why lighting is just dark and depressing rather than coloured at snooker venues

When you look at plants, which are typically green, the pigment comes from the chlorophyll in the leaves. Chlorophyll is responsible for producing the plant’s food and uses energy from sunlight to convert water and carbon dioxide into energy-rich sugars. Chlorophyll is green because it absorbs red light the best. Green plants actually photosynthesise optimally at red light – which is why they absorb it – and the least optimally at green light – which is why they reflect it.

So physical objects absorb and reflect light to determine the colour we see, but what about something less physical? Let us ask the question that most of us will ask when young but take for granted when old: Why is the sky blue?

The ages old question that we ask as a child and can't be assed to google as an adult

The age old question that we ask as a child and can’t be assed to google as an adult

Above I mentioned that different wavelengths of light interact with matter in different ways. When light comes into contact with the particles in the air, or even those that make up the air, different things happen depending on its wavelength. If the light’s wavelength is smaller than the molecule the light will be absorbed by the atom and then re-emitted, called ‘scattering’. This means that light with the smallest wavelength (the violet end of the spectrum) is scattered the best. This scattering means that the sky looks like a dome of blue, rather than patchy bits of blue with holes of infinite space showing through.

But, it might strike you as odd that despite indigo light being the colour that has the smallest wavelength, as so the colour that should be scattered the best, the sky isn’t indigo. This isn’t an inconsistency of the physics, but actually because of how our eyes interpret colour. They perceive colour through structures called cones. Our retinas contain five million cones each, with there being three types to perceive different types of colours. Our visual senses are not entirely accurate when it comes to colour, with certain combinations of light wavelengths sometimes being interpreted as an entirely different colour all together. So, the blue- violet light of the sky is interpreted instead as a combination of blue and white light, which is why the sky is light blue.


Hope you’ve found this article en-lightening!





Pictures are from here, here and here. Header picture is from here.

For further reading, have a look at my references here and here.

This article uses past writing from an assignment submitted to the University of Western Australia.



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