Colours of the world - Part 1 Science

Why the sky is blue and the leaf green

Chlorophyll is the most common among plant pigments; it absorbs blue and yellow light and reflects green. This is why most plant leaves are green. Photo: K.K.Mustafah   | Photo Credit: K_K_Mustafah

If you look up at the sky, you will probably see a vast blue expanse with some clouds scurrying around. But did you know that the light from the sun is actually white? Why does the sky appear blue, then?

Light from the sun is actually made up of all colours of the rainbow: violet, indigo, blue, green, yellow, orange and red, or VIBGYOR, as they are called together. When the sun shines and rain falls at the same time, each raindrop acts as a prism and “splits” white light into the component colours to form a beautiful rainbow across the sky. The rainbow is in fact made of all sorts of colours and shades from violet to red, and even beyond — colours that we cannot see with our naked eye.

The light we see is just a very tiny fraction of the light energy in our universe. Light is a form of energy, and travels in “waves” — like waves on the ocean surface. Light also travels in straight lines, unless something gets in the way. Mirrors reflect light, which is why you can see yourself in the reflection. A prism, or a raindrop, bends the light to reveal all the component colours separated out. The separation usually occurs according to the wavelength: violet has the shortest and red the longest wavelength in visible light.

Some molecules “scatter” light. When sunlight passes through the earth’s atmosphere, the molecules of gases in the atmosphere scatter the light; blue is scattered more than any other colours because of its smaller wavelength. And that’s why the sky is blue most of the time. Because the blues and violets are scattered away, the other colours together appear yellow, giving us the yellow sun we all put into our drawings. Out in space, the “sky” is black and the sun looks white. Why white? Because there’s no atmosphere to scatter the light!

But, when the sun starts going down, it’s actually further from you. Light is passing through more of the atmosphere to reach you. So much of the shorter wavelength light is scattered, that only long wavelengths are left in the light that reaches your eyes. Dust particles and water vapour reflect and scatter the reds and yellows, making the sun and the skies red or orange.

I am sure a lot of you have seen the sea. It sometimes looks blue, sometimes green and other times grey. On a clear day, when light strikes against water, the water acts like the atmosphere does: molecules filter out longer wavelengths and reflects blue. Hence, large bodies of water look blue. Water in your bottle, however, is almost colourless, or takes on the colour of the bottle. This is because for the water to reflect blue light, it has to be a large body of water, at least 1 metre in depth.

The sea also reflects the colour of the sky: the surface of the sea acts like tiny mirrors, reflecting the sky. On a cloudy day, you will thus see a grey sea. Some water bodies also have different colours according to what is suspended in them. A river with a large amount of mud in it, like after heavy rain, looks brown. In some seas, tiny plants called “algae” give colour. Most algae are green, but some are red.

Why are most leaves green, and how do some plants have leaves of other colours? Plants carry out “photosynthesis”, the process by which they use sunlight, carbon dioxide and water to produce carbohydrates. This is why they are called “producers”; us humans are consumers, we can’t make carbohydrates on our own. The reason why plants can capture light energy is because they have pigments that can absorb some wavelengths of light and reflect the others. The light absorbed is used for photosynthesis.

Chlorophyll is the most common among plant pigments; it absorbs blue and yellow light and reflects green. This is why most plant leaves are green. Plants also have “accessory pigments” that improve the efficiency of light harvest: they absorb light that cannot be absorbed by chlorophyll. Carotenoids, found in red/yellow fruits and vegetables like carrots and tomatoes, and anthocyanins, which give colour to almost all the beautiful flowers you see, are both examples. In temperate countries, when leaves fall during the autumn season, chlorophylls degrade into colourless substances and the accessory pigments show up in brown and yellow leaves.

How do we perceive colours? When light falls on an object, the object absorbs some of the light and reflects the rest of it. The wavelengths that are reflected or absorbed depends on the object. At the back of your eye is a sort of “screen” on which images form. There are special structures called “cones” that carry information from the retina to the brain, which decodes the signals and figures out the colour.

Humans have three types of cones: different colours stimulate different combinations of cones, to produce the numerous shades that we can see. We can see more colour than dogs, cats, cows — in fact more than a lot of animals. Some birds and fish beat us hollow when it comes to colours though; they have four types of cones, allowing them to see more colours. Bees, for instance, can see ultraviolet colours, which we can’t. You know what this means. The rose that seems red to you could look completely different to a bee or a dog.

Next week, we’ll look at the role colours play in the living world. How do different animals and plants use colour in their daily lives? Does it matter equally to a black crow and to a beautiful butterfly?

Sandhya Sekar is a science journalist who writes about weird and wonderful creatures. You can write to her at sandysek@gmail.com or read more on her website ‘The Melting Pot’ at www.sandhyasekar.com


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Printable version | Jul 26, 2021 2:17:49 PM | https://www.thehindu.com/in-school/sh-science/why-the-sky-is-blue-and-the-leaf-green/article6626954.ece

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