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Absorption and scattering of light: the interaction of light with matter

In the vast and complete vacuum, when light moves forward, although the energy will gradually disperse during the propagation process, the total amount is always constant without any loss. However, once light enters the realm of material media, its energy is often lost due to absorption and scattering effects.

The absorption phenomenon arises from the conversion of light energy into other forms. The most common is the conversion into heat energy, which increases the temperature of the object;

It can also be converted into radiation of another wavelength, such as fluorescence, which is the case with fluorescent materials that glow faintly in the dark;

It can also be converted into electrical energy. Photocells use this principle to convert light energy into usable electrical energy;

The photosynthesis of plants converts light energy into chemical energy, laying the foundation for life activities on Earth.

When a parallel light beam passes through a uniform medium, the light intensity loss of a specific wavelength light beam follows an exponential decay curve. Among them, represents the initial light intensity of the light beam, is the intensity of the light beam after the propagation distance in the medium, and is the linear absorptivity usually related to the wavelength. For highly transparent materials, the value is extremely small, and unless the propagation distance is extremely long, it is not much different from the value of. In many materials, the value is very large for all wavelengths, and the light intensity even approaches zero in a short distance. Such materials (such as metals) are usually opaque unless they are made into thin films. Some materials have significantly different absorption coefficients for different wavelengths in the visible spectrum. When light passes through, the spectral power distribution changes. This is the basis for the working of color filters.

Under certain conditions, the absorptivity of the medium can even become negative, that is, the intensity of light increases when it passes through the medium. This is the core principle of lasers. Lasers can produce extremely high-intensity beams, and the additional energy required to enhance the beam must be supplied to the medium by appropriate energy.

Scattering occurs in inhomogeneous media. It is caused by multiple reflections and refractions of light on many randomly distributed interfaces in the medium. Clouds and fog in the sky are typical examples of scattering caused by suspended water droplets in the air. After light enters a scattering medium, most of it will be scattered back, and the absorption loss is small, such as the surface of white paper and white cloth composed of dense and nearly transparent fibers. But when light encounters a scattering surface that strongly absorbs light, such as the carbon particles in black smoke, only a very small amount of light can be scattered, so this type of medium looks black. If the scattering particles selectively absorb some wavelengths of light in the visible light region, the medium will appear to be a specific color. For example, when white light enters the paint layer and then reflects back, the pigment particles suspended in the transparent paint absorb certain wavelengths of white light, making the paint appear colored. In addition, due to the effect of diffracted light, scattering may also be wavelength-selective, making the medium appear colored. From the perspective of the molecular structure of matter, all media will scatter light to a certain extent. Very small particles, such as molecules, scatter short-wave light more strongly, which also explains why a clear sky appears blue.

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