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Is Sky Really Blue

Color is one of the intrinsic properties of something material. We perceive its color because some light reflected from it reaches our eyes. The smallest unit of matter with specific properties is a molecule. All matter in the universe represents conglomeration of different types of molecules in diverse configurations. How light gets affected by material in its path depends on the size of the material.

There are four basic processes that light goes through when it falls on an object. These are reflection, refraction, diffraction, and scattering. Of these only scattering is of any consequence when tiny particles like molecules are involved.On a bright clear day the sky appears blue. This happens because the earth has a gaseous atmosphere in which oxygen and nitrogen are the dominant components. When the light from the sun hits the nitrogen and oxygen molecules it is scattered in all directions.

By light we normally mean the visible part of electromagnetic radiation. The light from the sun contains the seven primary colors from violet to red. The color depends on the frequency or the wavelength of the radiation. The wavelength increases from violet to red, while the frequency decreases. For particles comparable to the wavelength of light the scattering depends on the wavelength. The shorter the wavelength, the larger is the amount of scattering.

The human eye senses color mainly through three primary colors ? red, green, and blue. That is why in television signals these three colors are used for producing the color pictures. The blue component of the scattered light is almost ten times stronger than the red.

Even though the violet and indigo are scattered more than blue the eye senses mainly blue. Therefore when we look at the sky away from the sun the sky appears blue. This is, of course, true only on a clear day. If there are extraneous particles in the atmosphere as in conditions of haze, they scatter light differently and the sky does not appear blue. Also this is true only for observations on or near the earth. In space there is no atmosphere and no scattering particles.

There is no light coming to the observer except directly from the sun and the sky appears black or dark.In the literature we often come across the question: why is the sky blue? The use of the verb 'is' here is erroneous. The sky is neither blue nor black nor any color. As stated before, color is an intrinsic property of material objects and sky is not one. So we have another basic question: what is sky? The etymology for the word in dictionaries does not really provide any clues. The reference to Indo-European languages is also vague.

So we turn to Sanskrit, where the word for sky is aakaashah. It consists of two parts 'aa' and 'kaashah'. As a prefix 'aa' has several meanings, one of them is 'as far as'. The word 'kaashah' means visible. Thus akaashah means 'as far as visible'.

Taking the Sanskrit meaning sky means the empty space all around as far as one can see. Therefore the sky is nonmaterial and it cannot have any properties associated with matter.We see everything in the universe against the background of the sky, which is total emptiness. We do not see sky ? we cannot see emptiness.

What we see are the molecules of the atmospheric gases although not directly and in the normal sense of seeing objects. Everything is embedded in this emptiness. In fact, for any human perception it is necessary to have the background of a corresponding emptiness. To see the pictures on television we must have black or gray screen. To hear a sound we need the background of silence and so on.The sky has no shape and no size.

From the earth it appears as a hemispherical dome because the earth is spherical. It extends as far as we can see, which is the limit of the universe. But what lies beyond? Nothing.

And what happens when all the matter in the universe vanishes? Space-time also vanishes and nothing remains. The emptiness represented by the sky then merges with the all-pervading emptiness described by ancient philosophies and experienced in deep meditation. At the microscopic level it may be likened to the quantum void, which is empty but holds the potential for all matter to appear. In Vedanta and Buddhism the word shunyata is used for this emptiness. It pervades all, contains all, and transcends all.

.Dharmbir Rai Sharma is a retired professor with electrical engineering and physics background. He obtained his M.S. degree in physics in India and Ph.D.

in electrical engineering at Cornell University. He has taught in universities here and also in Brazil, where he spent sometime. He maintains a website http://www.

cosmosebooks.com devoted mainly to philosophy and science.

By: Dharmbir Sharma

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