Why is the sky blue... et al.

We all know the sky is blue. It is pretty obvious isn't it? But three days back, when my son added a 'why' to that pretty obvious statement, and looked at me with big curious eyes, I realized that I didn't know the answer to a question about something that has been over my head all my life. How ignorant could I  have been!! The conversation went like this ..

My 5 year old son : Ma, today it's so hot. See how blueeee the sky is. But ma, why is it blue?
I : It's blue because... because.... ( oh dear, I don't know.. why don't I know... and as an attempt to save face I end up saying ) .... hmm .... baby, what did you learn in school today. Tell me, what did dhiya and prithvi tell you today?
Son : Ma, school was fine. My friends are good. Will you look it up on 'gggooogle' and tell me later?
I :  .....

And the very second I got some free time, I picked up my phone and 'googled' - "why is the sky blue" ... And I read extensively.  One thing led to another and I learnt a lot of fascinating things. ( Don't worry, I think i'll split all the fascinating things into two posts) 

Before we even get on to the why is the sky blue , let's think on this first :  What governs an object's colour ? What if someone asked you,  "Why is a banana yellow?", what would you say? Take a break now, compose your answer and then read ahead. The best answer I could've come up with would've been ,"because.. its skin has yellow pigment". That's all. But, what is that yellow pigment? Why certain pigments and dyes give off certain colour. I drew a blank.

Time to put on our thinking hats (I was tempted to say sorting hats, coz it's the Harry Potter season.. But hey, let's focus ). Keeping the color aside for a moment, what do we need inorder to see things ? We need light to fall on the object and illumiate it. We need an eye to interpret the light that reaches it after illuminating the object. And goes without saying, (as the eye is doing the interpretation), the light has to be in the visible range of the spectrum. So when visible light falls on the banana, and reflects off it and reaches my eye, my eye sees an image of the banana and projects it to the brain. 

And right in between those sequence of events, ( after the light reaches the banana,)  the banana absorbs all the colors of visible light except yellow, thereby reflecting off yellow light, that alone reaches the eye. The brain with the help of the eye, interprets the colour to be yellow. Hence, the yellow banana. Which inevitably brings us to the next question. Why does banana reflect yellow and absorb every other colour? ( when will these nested why questions end? This is how it must feel inside my son's head)

Rather, why do certain objects absorb certain colors and reflect certain other colors. When light falls on an object, the light is either absorbed, reflected or transmitted through. Who or what do you think decides the fate of the incident light? It is the atoms or molecules that make up the object.

Objects are made up different types of molecules and molecules maybe made up of different types of atoms. Atoms could be further broken down into protons electrons and neutrons . Our focus now being on the electrons which are not so tightly bound to be nucleus. Let's say it's like binding a ball to a point with a spring. So the electrons can move up and down. And electrons tend to have their natural frequency of vibration. This natural frequency is the signature of the electrons in the particular type of atom. 

Now each colour from the visible spectrum has a frequency of its own. If the frequency of the colour matches with the frequency of vibration of the electrons in that particular object, that excites the electron and sets it vibrating. As it moves , it collides with the other atoms and the energy is converted into heat/thermal energy. No light comes out. So we could say that particular colour was absorbed by the object.

If the incident coloured light's frequency doesn't match the natural vibrating frequency of the electrons, the electrons don't vibrate for long, but vibrate for a brief period and sometimes transmit the vibration to the neighbouring electrons. The vibration is passed along, and if this goes on, the light is transmitted out at the other end of the object.

However, sometimes, the energy is so insufficient that the electrons on the surface of the object vibrate for a brief period of time and re-emit the energy out of the surface, causing reflection. 

And because every type of an electron and hence an atom has an inherent frequency of vibration, it behaves uniquely when visible light falls on it. Some of them reflect and transmit certain colour lights. Would that imply, that we are equipped to answer the 'what is a pigment' question now? Yes, of course.  Pigment is a chemical that is capable of absorbing some colors and reflecting one or more of the colors in order to lend a certain shade to a certain object. Voila. Neat isn't it !

Back to the banana... Looks like the atoms in the banana absorb every other colour and reflect only yellow. Hence the colour. If all colours would've been absorbed, it would appear black. If all colours were transmitted, it would be transparent. If all the colours were reflected, it would appear white. Get the hang of it?

Wwooh... What a long piece of text before getting to 'the' question : why is the sky blue? Imagining sky as an object and extrapolating from what we've just learnt so far, you would be tempted to say, the atoms in the atmosphere absorb every other colour except blue. Hence the color blue. Right ? Actually, it's a bit more complicated than that.. Don't fret. The knowledge you have just acquired after reading so far is not wrong.( I agree, it sure is hard work. ) Just that it is a different story when it comes to the color of the sky. 

What should we know to be able to understand this story? One, the visible light is made of 7 colors and that each of that color has a wavelength of its own. (violet to red, in ascending order of wavelength) Two, the earth's atmosphere is made up of nitrogen and oxygen molecules, molecules which are very small. Both those sound trivial isn't it ( like my son puts it : easy peasy lemon squeezy ) 

When the light enters the atmosphere, all the other colors travel unhindered, however blue light which has small wavelength is scattered by the  small oxygen-and-nitrogen-molecules. The scattered blue light when it reaches the eye, lends the blue color to the sky. There we go !!!. (Save the question that might be bothering you now, for just a little while longer ) 

That explains the blue sky during the day. However, during Sunrise and sunset, the sky gets a nice red-orange tinge. How is that possible now? What happens to blue light then? During the sunrise or the sunset, the sun is far away from the earth ( as compared to where the sun is during the day) . So the light of the sun gets to travel longer and while doing so all the blue light is totally scattered and the remaining red and orange reach our eyes giving us the wonderful red hue.

Ding dong!! Time for the saved question. If the nitrogen and oxygen molecules affect only the low wavelengths , then what about violet ? Why isn't our sky violet or even blue-ish violet or violet-ish blue? Violet has an even smaller wavelength. Right ? Puzzling!! There's a two part answer to this question. I saw a very colorful graph while investigating this over the Internet about the proportion of the colors in the sun's light. Graphs rattle me a bit, so I'm going to leave it out of here. All it said was, there is less violet than blue in the light that comes from the sun. That's reason one. Reason two revolves around how our eye perceives colour. Apparently, our eye is more sensitive to blue than violet. 

The Retina in our eye has cells called cones that perceive color. And we have three types of cones, blue, green and red which are most sensitive to blue, green and red respectively. However each cone responds to a range of wavelengths. Again, there was a graph to explain that .. I'm skipping the graph. All you need to know is, the violet and indigo lights excite the blue and a little bit of red cones too. The blue light excites the blue as well and little of green cones too. So, when we look up at the sky, lot of blue cones get stimulated, some of green and some of red get simulated. Giving you the combination of blue and little white, the sky blue . Tadaaa!!! ( some red and some green and some blue, gives some white )

So, if the blue color of the sky is perceived in our eyes, it could very well mean that the animals and birds with a different eye structure could see the sky in a different color. How cool is that? A whole new different world out there for them. To support that theory, Did you know that when a honey bee looks at a flower, it can see something called nectar guide. That is, the portion of the flower that contains the nectar shows up highlighted. And that's because the honey bees can see ultraviolet. Buzzing brilliant I would say!! The beautiful ways of nature. 

From why is the sky blue, we have travelled a long way and reached a crossroad called "Is it really blue?". Well, you know what they say ," Color lies in the eye of the beholder".