Now I will put it to you in this way. The best way to answer a question is to ask another. At night, we all see the stars. On a fairly clear night you see the stars twinkling in the sky. Why are the stars not visible in day time? Please ask yourself this modest question. Well, the reason obviously is that the earth, as a lady, has hidden herself under a veil.
The sky is a veil which she has thrown around us. We cannot see the stars during the day, because the veil hides the stars. And what is this veil? The veil obviously is the atmosphere of the earth. The same veil which at night is so transparent that we can see the faintest star and the milky 2 way, is covered up in day-time. Obviously, it is the atmosphere which is the veil. And we see the sky as blue only because we have not got other thicker veils like these clouds.
You see for example, those clouds high in the blue sky. Obviously therefore, for the sky to be really blue, there must be nothing else, no clouds and perhaps no dust. The clearer the sky is, the bluer it is. So the sky is not always blue; it is sometimes blue and sometimes not blue at all.
So that the mere looking at the sky enables us to understand the condition of the atmosphere. Let me say one thing more. Obviously, the sky and the atmosphere are lit up by the sunlight. Sunlight is passing through this great column of air and obviously it is the atmosphere, something that is transparent and invisible at night, that is seen to us by the light - sunlight - passing through the atmosphere.
Now I want you to ask yourself another question. You know that moonlight is only the sunlight incident on the moon and is diffused or reflected. You will be astonished to find that the sky is not blue. It appears pale, you just see some light and you see some of the stars even under the full-moon sky, but the sky is not blue. Why is it that the sky which appears blue in sunlight, does not appear blue in moonlight? The answer obviously is : the illumination is far less powerful.
I present it to some young mathematician to sit down and work out. How big is the moon? What should be the brightness of moonlight?
It is a little astronomical problem. Rough arithmetic would tell you that moonlight is something like half a millionth part as bright as sunlight; you would think, it is terribly small. But moonlight, when it is there seems very bright though it is only half a millionth part of the brightness of sunlight.
Why does it look so bright? Well, the eyes have got accustomed to much lower levels of illumination. So moonlight appears very bright but 3 not so bright, as to veil all the stars. But the sky, it does not appear blue. So this comparison of sunlight and moonlight brings to our notice a very remarkable fact.
It is an absolutely fundamental aspect of human vision that to perceive colour, you must have a high level of illumination. This is a principle which perhaps is not so widely appreciated as it ought to be. Colour is only perceived at high levels of illumination. The higher the illumination the brighter are the colours. You go down to low levels of illumination, say, a millionth part, half a millionth or a hundred thousandth part of sunlight, the sense of colour disappears.
I can go on giving any number of illustrations. Perhaps the most striking illustration emerges when you look at the stars or such objects as the Orion nebula through small telescopes. Let me say here and now, my belief that there is no science so grand, so elevating, so intensely interesting as astronomy. It is amazing to see how many people high up have never seen the sky through the telescope. I want to tell them something which is absolutely incredible : Nothing more than a pair of binoculars, a good pair of binoculars is needed to educate oneself in the facts of astronomy.
I think a man who does not look at the sky even through that modest equipment - a pair of binoculars - cannot be called an educated person, because he has missed the most wonderful thing and that is the universe in which he lives.
You must have a look at it. I come back now to the problem of the blue sky. I want to pose to you a very difficult question. Why is it that we perceive the blue colour only under intense illumination in sunlight, and not in moonlight?
I will bypass that and come back to the question : Why is the sky blue? Well, we all know that white 4 light is composed of all the colours in the spectrum. You divide white light into various colours; you start with deep red at one end, light red, orange, yellow, green, blue and violet, so on, the whole range of colours. When I look up at the sky, I see only the blue; what has happened to the rest of the spectrum?
This is the basic question. The question becomes a very pressing one when 1 remark that when we actually spread out sunlight into a spectrum, the blue part of it is the least intense part. It has simply vanished. It is not there at all. You can look very very hard and try to see if you can see any red or yellow or green in blue sky. The blue has just masked the rest of the spectrum. This is a very remarkable fact.
If you watch the sky on some occasions, you get great masses of white clouds, what they call, the cumulus clouds not huge things, just little bunches. It is a beautiful sight to see the blue sky and these little masses soaring above.
I have derived great satisfaction in just doing nothing at all and looking at these masses of clouds and the blue sky. The interesting point is precisely when you have the clouds moving about that the sky is bluest.
What it means is that these cumulus clouds in the course of their formation just cleaned up the rest of the atmosphere. They take up the dust particles and concentrate them on the white clouds.
The rest is left nice and clean. You see the beautiful blue view against the brilliant white, it is a very lovely sight. You may ask me, how is the cleaning process accomplished? Now here is a wonderful story. The usual answer you get is that the cloud is steam, but it is nothing of the sort. The cloud consists of particles and what looks to us as great masses of white clouds are just droplets of water.
Water is heavy but why does it not fall down? We find it floating in the air! You see that is another problem. Already 1 am going from 5 one problem to another. We ask ourselves, what is a cloud?
Why is it floating in the air? Now the interesting point is this you cannot have a cloud unless you have dust particles about which it can form. There must be particles of some sort, may be very small, may be very large. If there is no dust in the air, there will be no cloud and no rain. You see, how from the blue sky, we have got on to the origin of rain, rain fall and so on.
One thing leads to another. That is the essence of science. You must go deeper where it leads you. You cannot go thus far and no further. The moment you raise a question, another question arises, then another question, so on and so on. Ultimately, you find that you have to travel the whole field of science before you get the answer to the question : Why the sky is blue?
So I told you this fact about the clouds. Well, I should say the cloud cleans up the atmosphere. Cloud forms and then leaves the atmosphere clean, comparatively free from dust particles and other nuclei and that is why the sky is blue. So we come down at last to getting some kind of answer to the problem. The sky is blue because the atmosphere is clean and free from dust and all nuclei.
The clearer it is, the bluer it looks, provided there is enough light. So you come somewhere near the answer to the question.
What is it you are able to see? The fact is that when we see a blue sky, we see the atmosphere of the earth, the gases of the atmosphere, they diffuse the light and we see the blue light of the sky. But still we are far from the answer.
What happens to the rest of the light, the sunlight? That is the question. Now this question can be answered in the following fashion. Raman is actually speaking, moving his hands about and forcefully making his points. The pictures, the talk, the introduction, a crisp timeline listing milestones, the anecdotes, and a simple explanation of the Raman Effect all combine to capture the essence of a multifaceted man who was an inspiring example of the scientific spirit.
In this book, Dr. Raman says that science is not restricted only to laboratories — science to him comes from nature. It is about questioning deeper and about being in the state of wondering; not expecting short answers.
One does not need advanced equipment to experience science. All that is required is a thirst for knowledge. View larger. Shop By Language. On my wishlist Add to bag. Notify me when available. Raman, love for science, science and children, natural phenomena, educational, colour Dr C. Makes science look easy and interesting In this book, Dr. Write a review.
Gitti the rock is very Eventually some of it scatters down to our eyes and makes the sky appear blue. The remaining, non-scattered light is yellow or orange, and this is what we perceive as the light coming directly from the sun. When the sun is parallel to the Earth, none of the Tyndall-scattered blue light reaches our eyes at all—we see only the red light left over after the rest has been scattered.
Not all the technical details of this theory are correct. It turns out that sunlight is not pure white light, but closer to a blackbody spectrum. The particles that the sunlight scatters off of are not dust particles, but rather pockets of hotter or cooler air, which act like particles due to refraction.
And the fact that we see the sky as blue, rather than violet an even shorter wavelength of light that experiences even more scattering , has more to do with how the human eye evolved than anything special about blue light itself. Part 2: Lord Rayleigh. But why does blue light scatter more than red light? And, for that matter, how does scattering work at all? In the time since Tyndall, James Clerk Maxwell had discovered that light is made of electric and magnetic fields.
For a more detailed description, see my article on refraction. Maxwell discovered that these fields can feed into each other and become self-sustaining. A changing electric field produces a magnetic field, which produces an electric field when it changes, and so on. Moreover, when these fields oscillate like this, they behave exactly like light—meaning that light is a wave made up from these fields!
I am, of course, glossing over the fact that light is both a particle and a wave. From the quantum perspective, the electromagnetic wave describes the probability of detecting a photon at a given place and time. Rayleigh also knew that an atom is made up of a positively- charged nucleus surrounded by negatively-charged electrons. As we know from Bohr , this is essentially correct.
What would happen if you were to somehow pull one of these electrons away from the nucleus? But because it keeps overcorrecting for the perturbation, the electron yo-yos back and forth between two elliptical orbits. Wobbling the electron costs energy, which is taken out of the electromagnetic field, causing the incoming light to be absorbed by the atom and disappear.
And since the electron is a charged particle, this tracing-out actually recreates its electric field—producing more light of the same color as the original! To reiterate: The electron absorbs the original light, then re-emits it in a random direction.
We call this behavior Rayleigh scattering. So why is the scattered light more likely to be blue? And because blue light has a shorter wavelength than red light, it accelerates the electrons more quickly, which makes them more likely to absorb light.
I will explain resonance sometime in the future, I promise! Part 3: Adolf Smekal and Sir C. Rayleigh gave us an explanation for how light scatters off of atoms. But what about molecules? And in , the brilliant experimentalist Sir Chandrasekhara Venkata Raman verified the effect.
His discovery won him the Nobel prize. In an atom, electrons are localized to one nucleus. But in a molecule, the electrons have several atoms to roam across.
As I discussed in my post on bonding , atoms in a molecule share electrons. When an electromagnetic field comes along, it pushes the electrons into preferred positions, which causes the molecule to polarize —meaning that certain parts of the molecule are positively charged and other parts are negatively charged.
After all, our electromagnetic field moved the electron then, too. But atomic bonds in molecules are not static things. Because of the heat in the molecule, the atomic bonds wobble and vibrate all on their own.
This means that once a molecule is polarized, the electrons wobble, too! So what happens to incident light? Well, the wiggles of the electromagnetic field do indeed wiggle the electrons.
Thus, the wiggle that the electrons trace out to produce the new outgoing light is different than the wiggling of the incoming light alone. As a result, the scattered light can be a different color—either a higher or a lower frequency—than the original light.
Applications for Raman Scattering. Graphene is amazing stuff, by the way…amazing enough that you should expect a whole post on it at some point. A small brag: these Raman spectra plots are actual data taken by me when I was an undergraduate student. The measured graphene was even grown by me in the lab.
If the atomic bonds change, the Raman spectrum can track that, too.
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