Transparency And Color

You have probably wondered, at some point, why some materials are transparent, notably water and glass, while most are opaque and do not allow light to pass through.

The fact that light slows down when it goes from a medium with a low refractive index, such as air, to one with a higher refractive index, such as glass or water, means that lenses can be made and an object under water is not actually located where our eyes perceive it. But that opens another question; if the speed of light is so fixed by relativity theory, then how can it slow down when it enters another medium?

Air is transparent to light for a very simple reason. It is sparse enough to allow light to pass right through. Air is actually heavier than water by molecule, water has a molecular mass of 10 while the diatomic oxygen that is in the air has a mass of 16 and diatomic nitrogen of 14. But yet when water molecules are held together by the hydrogen bonding that occurs, it forms the familiar liquid and is 800 times as heavy as air at sea level. This means that the molecules in the air must actually occupy only about 1/1200 of the total space.

But then how does glass allow light to pass right through? The answer to this is also simple. Glass is a crystal in it's molecular structure. The chemistry of glass is actually similar to sand but it's molecules are lined up in rows and light can pass through the gaps between the rows since atoms are roughly spherical in shape. Imagine cans of soda stacked up with the cans aligned, light can pass through the gaps between the cans.

I must disagree with the popular idea in physics that light slows down when it goes from air into glass or into water, that would violate relativity. My explanation for this apparent deceleration of light that makes lenses possible is that while light is passing through the gaps between atoms in a crystal structure, there will not be a straight line for the light all the way through a thickness of thousands of millions of atoms For this to happen the glass would have to be at a temperature of absolute zero to eliminate molecular shifting due to heat.

Since the gaps between the atoms cannot possibly form straight lines all the way through, lot of refecting off the sides of the gaps is involved when light passes through a dense, transparent medium. This makes the path of the light longer than it would be otherwise and this makes light seem to slow down as it passes through glass. The index of refraction of a transparent medium is thus a function of the length of the path light must take to get through it. This also means that even clear glass must scatter light passing through it to a considerable extent but this scattering is over such a small scale that our eyes cannot detect it.

The reason that a glass prism splits light into it's component colors (colours) now becomes apparent. Ordinary white light is actually a mixture of all visible colours (colors) with that of the longest wavelength being red and that of the shortest wavelength being blue. The light of the shorter wavelength is more "compact" than the longer wavelengths of light. This means that in relation to blue light, the gap between the atoms will be relatively wider than it will be to the red light. So blue will do more reflecting off the sides of the gap to keep it in course while with the longer-wavelength red light, the gap will act more as a waveguide with less reflection involved.

The result is that the shorter the wavelength, the more the light will be bent by the structure of the glass and so white light will be broken down into it's component colours (colors). This will happen only if the white light is in a concise beam and it enters the glass at an angle to the surface.

What about water? It is also transparent. The reason is it's polar structure. A water molecule, consisting of an atom of oxygen joined to two atoms of hydrogen has an unsymmetrical structure. This makes it more negative on one side and more postitive on the other side so that water molecules line up negative side to positive side. This forms, in effect, a crystal structure with gaps between the molecules through which light can pass in the same way as it does through glass and other crystal materials.

By the way, you may notice that the dissolution of air in water does not affect it's transparency at all. You can see through clear shallow water regardless of how little or how much oxygen or CO2 is dissolved in it. If this were not so, the transparency of water would be affected by it's temperature since this determines the volume of gases that can dissolve in it. The reason is that atoms of dissolved atmospheric gases fit into the matrix of the structure of the atoms in the water. This supports the findings that I described in "The Collision Imbalance And The Evaporation-Dissolution Exchange" on my meteorology blog. If the atoms of dissolved gases were floating around in the water without fitting in to it's "crystal" structure, it would affect the transparency of the water.

Finally, we come to the colors (colours) of opaque (non-transparent) objects. Objects without the crystalline atomic structure required for transparency will not allow light to pass through. But it handles various wavelengths of light differently. The color (colour) of an object that we see is the result of the size and the space between it's surface atoms. That light with wavelengths short enough to fit between the atoms will be swallowed up by the object and will not be seen. The wavelength of light that we see will be that which the surface atoms reflect without absorbing or scattering.