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# Properties of light

Light has different properties that need to be studied, but first we need to understand:

#### What is light?

Light is electromagnetic radiation that has properties of waves.  The electromagnetic spectrum can be divided into several bands based on the wavelength. Visible light represents a narrow band of wavelengths that range between about 380 - 730 nm. These wavelengths are interpreted by our eyes as different colors.  If a single wavelength or limited range of wavelengths are observed, they are interpreted as a definite color.  A single wavelength is defined as a monochromatic light. All wavelengths of visible light together are interpreted as white light by our eyes. Dark is interpreted when no wavelengths in the visible range are present.   Light electromagnetic radiation is within a certain range of the electromagnetic spectrum. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), between the the longer wavelengths infrared and the shorter wavelengths ultraviolet.

1. #### Velocity of Light and Refractive Index

The relation between light energy, the frequency and velocity is defined as follows: E = hν = hC/λ where E = energy h = Planck's constant, 6.62517 x 10-27 erg.sec ν = frequency C = velocity of light = 2.99793 x 1010  cm/sec λ= wavelength The velocity of light in vacuum is 2.99793 x 1010cm/sec.  Light cannot reach faster speed, and its velocity will decrease when travelling through matter. Since C = νλ The frequency of vibration,ν, stays constant when the light is passing through a substance.  Thus, if the velocity of light, C, is reduced when passing through any substance, the wavelength, λ, must also decrease. Thus refractive index, n, of a material is defined as the ratio of the speed of light in vacuum, C, to the speed of light in a material through which it passes, Cm.   n = C/Cm The refractive index value will always be > 1, since Cm can never exceed C.  Generally,  Cm depends on the density the material, and  Cm decreases with increasing density.  Therefore, the higher density materials will have greater refractive indices. Refractive index changes linearly with wavelength.  The refractive index the material relies on the light’s wavelength, as different wavelengths are inhibited to different extents by the atomic structure of the material.
1. #### Reflection and Refraction of Light

When light falls on an interface between two substances with different refractive indices, two things occur. The incident ray of light striking the interface at an angle - measured between a line perpendicular to the interface and the propagation direction of the incident ray- will be reflected off the interface at the same angle and another portion can be refracted by passing through the other medium - if also transparent- but with a changed direction.
1. #### Dispersion of Light

Dispersion is produced by the fact that refractive indices vary for each wavelength of light.  This is observed by shining a beam of white light onto a triangular prism made of glass. White light incident on the prism will be refracted through the prism by different angles which depend on the wavelength of the light
1. #### Absorption of Light

When light passes through a transparent material  a part of its energy is dissipated as heat energy, thus losing some of its intensity.  When this absorption of energy selectively occurs for some wavelengths of light, the light that is transmitted through the material will be composed only of the wavelengths of light that are not absorbed.  The wavelengths transmitted will then resemble a color, called the absorption color of the material.
1. #### Polarization of Light

Normal light equally vibrates in all direction perpendicular to its path of propagation. Light is called plane polarized if the light is allowed to vibrate in only one plane with the vibration direction being the direction that the light vibrates Polarization of Light
Finally ,the characteristics that define behavior of visible light are: (Intensity, Propagation direction, Frequency or wavelength spectrum, Polarization and Speed in different material)
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