38. Dispersion of light.

Dispersion is the splitting up of white light into seven colors on passing through a transparent medium like a glass prism. When a white light beam is passed through a prism, a band of seven colors are formed is known as spectrum of white light.The seven colors in the spectrum are red, orange, yellow, green, blue, indigo and violet respectively. The formation of spectrum of seven colors shows that white light is a mixture of seven colors (or seven colored light). The effect of transparent medium (like glass prism) is only to separate the seven colors of white light. White light is a mixture of lights of seven colors: red, orange, yellow, green, blue, indigo and violet. The dispersion of light occurs because the angle of refraction (or the angle of bending) of lights of different colors is different when passing through the transparent medium (glass prism). So, when white light consisting of seven colors falls on a transparent medium (glass prism) , each color in it is refracted ( or deviated ) by a different angle , with the result that seven colors are spread out to form a spectrum, The red color is deviated least , so it forms the upper part of the spectrum . On the other hand, Violet color is deviated the maximum, so violet appears at the bottom of the spectrum. Example: The rainbow is an arch of seven colors visible in the sky which is produced by the dispersion of sun’s light by rainbow drops in the atmosphere. The rainbow is actually a natural spectrum of sunlight in the sky. A rainbow is produced by the dispersion of white sunlight by raindrops (or water drops) in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum.

Chromatic dispersion can be a problem in optical equipment like cameras, microscopes and telescopes. Since a lens is similar to a prism, it will disperse the light into a spectrum. This can be exaggerated when there are several lenses in the system. But you don't want a blurry image where the system focuses different colors of light at different spots. Such blurring is called chromatic aberration.

The solution to that problem is to combine a positive lens that has one index of refraction with a negative lens of a different type of glass and a different index of refraction, such that the dispersion of the two negate each other.

39. Absorption of light. Buger’s law.

The absorption of light occurs when a ray of light strikes a surface. The energy from the light is transferred to the surface material. The transfer creates heat (usually small amounts). An absorbing surface prevents reflection or diffusion of light striking on the surface. In practice, perfect absorption does not exist. However, the surface of photographic absorbers are normally of low reflectivity as a result of texture and colour.

A highly absorbing surface, like the matt black of a blackboard, has a higher transfer of light to energy at the surface. Consequently the board will become warmer or hotter than the ambient temperature in strong direct sunlight. Equally a mirror with a very high reflection level will absorb very little energy and will remain close to the ambient temperature.

Buger’s law. The law is commonly applied to chemical analysis measurements and used in understanding attenuation in physical optics, for photons, neutrons or rarefied gases. In mathematical physics, this law arises as a solution of the BGK equation. The law was discovered by Pierre Bouguer before 1729.^[1] It is often attributed to Johann Heinrich Lambert, who cited Bouguer's Essai d'optique sur la gradation de la lumière (Claude Jombert, Paris, 1729)—and even quoted from it—in his Photometria in 1760.

By definition, the transmittance of material sample is related to its optical depth τ and to its absorbance A as

T=Φ^t_e/Φⁱ_e=e^-r=10^-A

where

• Φ_e^t is the radiant flux transmitted by that material sample;

• Φ_eⁱ is the radiant flux received by that material sample.

Buger has established Law of absorption of monochromatic light in a homogeneous environment for a parallel bunch:

The identical part of a stream of energy of a falling light wave is absorbed each layer of the environment of identical thickness, irrespective of its absolute size.