Color is one of the intrinsic properties of something material. We perceive its color because some of the light it reflects reaches our eyes. The smallest unit of matter with specific properties is a molecule. All matter in the universe represents a conglomeration of different types of molecules in various configurations. The way light is 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 has any consequence when tiny particles such as 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 sunlight hits nitrogen and oxygen molecules, it scatters in all directions. By light we normally understand the visible part of electromagnetic radiation. Sunlight contains the seven primary colors from violet to red. The color depends on the frequency or 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 greater the amount of scattering.
The human eye perceives color primarily through three primary colors: red, green, and blue. That is why in television signals these three colors are used to produce color images. The blue component of the scattered light is almost ten times stronger than the red. Although violet and indigo are more scattered than blue, the eye perceives 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 foreign particles in the atmosphere such as in hazy conditions, they scatter the light differently and the sky does not appear blue. Also this is true only for observations on or near the ground. In space there is no atmosphere and scattered particles. There is no light reaching the observer except directly from the sun and the sky appears black or dark.
In literature we often come across the question: why is the sky blue? The use of the verb ‘is’ here is wrong. The sky is neither blue nor black nor any color. As stated before, color is an intrinsic property of material objects and the sky is not. So we have another basic question: what is heaven? The etymology of the word in dictionaries doesn’t really provide any clues. The reference to Indo-European languages is also vague. So we turn to Sanskrit, where the word for heaven is aakaashah. It consists of two parts ‘aa’ and ‘kaashah’. As a prefix ‘aa’ has several meanings, one of which 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 around as far as one can see. Therefore, the sky is not material and cannot have any properties associated with matter.
We see everything in the universe against the background of the sky, which is a total void. We do not see the sky, we cannot see the void. What we see are the molecules of atmospheric gases, although not directly and in the normal sense of seeing objects. Everything is embedded in this void. In fact, for any human perception it is necessary to have the background of a corresponding void. To see the images on television we must have a black or gray screen. To hear a sound we need the background of silence and so on.
The sky has no shape or size. From the earth it looks like a hemispherical dome because the earth is spherical. It stretches as far as the eye can see, which is the limit of the universe. But what is beyond? Nothing. And what happens when all the matter in the universe vanishes? Space-time also vanishes and nothing remains. The void represented by the sky then merges with the omnipresent void described by ancient philosophies and experienced in deep meditation. On a microscopic level, it can be compared to the quantum vacuum, which is empty but has the potential for all matter to appear. In Vedanta and Buddhism the word shunyata is used for this void. It pervades everything, contains everything and transcends everything.