Propagation of light

Propagation of light refers to the manner in which an electromagnetic wave transfers its energy from one point to another. Three main processes generally occur when light passes between boundaries from one medium to another:

• Transmission
• Reflection
• Refraction

Propagation of light through vacuum

Little or no scattering occurs, hence a beam of light through vacuum will be completely invisible except for objects in the path of the light rays.

Propagation of light in gaseous media

(Transparent, not dense, with molecules randomly arranged.)

An electron at ground state absorbs a photon of a certain amount of energy. This energy sets the electron vibrating about the ground state without any excitation to the next higher possible energy level, as the energy is not the same as the energy difference between any 2 allowed quantum energy levels of the gas. Immediately, this electron re-emits another photon of the same energy in a random direction. This process occurs over and over again as the light waves meet each gas molecule, scattering the light in directions other than the original direction of propagation (laterally scattered) hence making the beam of light visible. But since the gaseous medium is not dense, only a small amount of light is laterally scattered, most of the energy will propagate through keeping the medium transparent.

Propagation of light in solids

(Transparent, more dense, with molecules orderly arranged.)

The scattering of light occurs and the re-emitted photons interfere to favour forward propagation. (In propagation through gas, interference does occur but due to the random arrangement of the molecules, no significant or noticeable pattern surfaces.) The order of the molecules causes most of the lateral and backwards scattering to interfere destructively and the forward scattering interfere constructively. Hence the overall effect of scattering enhances forward propagation, making propagation in solids more efficient than in gases.

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