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Light and MatterLight and Matter

What is the color spectrum?

The modern understanding of color originated in the discovery of the spectral nature of light by Isaac Newton in the 1600s. Newton's famous experiments demonstrated that light consists of energy of different wavelengths. We now know that the eye is sensitive to a broad band of wavelengths in the approximate range 350-750 nm. The visible spectrum represents only a small fraction of the full electromagnetic spectrum. Within the visible spectrum certain wavelengths give rise to certain visual sensations. For example, the shorter wavelengths are perceived to be violet and blue.

What happens when light strikes the surface of a material?

When light strikes a surface there are two things that can happen:
(i) the change in refractive index can cause light to be reflected by the surface and this surface-reflected light is called specular reflection;
(ii) light that is not reflected at the surface can penetrate the body of the material although as it passes through the surface the change in refractive index will cause the light to be refracted.
Light may pass completely through a material, in which case we say that it has been transmitted. Alternatively the light may be absorbed by the material or it may be scattered. Light that is scattered or reflected may eventually pass out of the front, back, or side of the material.

How is light absorbed?

Light can be absorbed by materials according to a number of mechanisms that include atomic vibrations and rotations, ligand-field effects, molecular orbitals, and charge transfer. It is very often the case that specific quantities of light (energy) are absorbed by a specific material and thus the light absorbtion properties of materials are usually wavelength selective.
The energy that is absorbed by molecules can be dissipated as kinetic and heat energy, but sometimes the energy can be re-emitted. Fluourescence and phosphorescence are phenomena that result from the re-emission of absorbed light energy: in both cases the re-emitted energy is at a longer wavelength than the light originally absorbed.

How is light scattered?

When light strikes particles it may be scattered. When the scattering particles are extremely small (the order of 1000 nm) the light is scattered according to a simple law proposed by Rayleigh: short wavelengths are scattered more than long wavelengths. For larger particles (the order of 4000 nm and larger) the amount of scattering is according to Fresnel's equations: the amount of scattering depends upon the difference between the refractive index of the particle and of the medium in which it is dispersed and this difference is wavelength dependent. If light is scattered evenly in all directions this is called isotropic scattering but it is rarely the case. The absorption and scattering properties of particles are complex and a number of theories exist to describe them including the Kubelka-Munk theory.

Why are some substances colored?

There are many reasons why substances appear colored but for most physical materials it is because the absorption and scattering properties of the material are different for different wavelengths of light. Thus a substance that appears yellow may do so because it absorbs most strongly in the blue part of the spectrum and scatters most strongly in the red and green parts of the spectrum. It is often the case that a pigment scatters light most efficiently in one region of the spectrum whilst having its main absorption band in another. This explains why translucent and transparent colored films can have different hues when viewed by reflected as opposed to transmitted light.
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