## Light attributes

Light perceived in human eye, from the physical point of view, is a narrow part in electromagnetic spectrum. Electromagnetic wave motion is given by length of period or frequency. It represents frequencies in the order 10^14 Hz. Visible light is in band from 380 nm (violet light) to 780 nm (red light). The area below 380 nm is ultraviolet radiation and above 780 nm is infrared radiation. The following laws are applicable on light movement:   :

 The law of linear movement of light The law of mutual independence of rays The law of reflection The law of refraction

Huygens' principle is applicable to light spreading as wave front. 'Light ray' is defined as a light trajectory between two points. Along a light ray are spread two wave motions: one, along the electric vector E; and the other, along the magnetic vector B; these are perpendicular to each other, while their phase is equal.

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Applet Snell's law

The law of refraction of a ray passing through two various environments is Snell's law:

 sin alfa       v1       n1     -----------  =  ----  =  ----     sin beta      v2       n2

Where   v1,v2 are velocities of light in environments, and values n1, n2 are refractive indexes for the given environments, and angles alfa and beta are the angle of impact and the angle of refraction. The index nv gives the rate of velocities in a vacuum and in the given environment. For the spreading of light in gases, this index is about 1, for glass it is 1.6, and for diamond is more than 2.

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Applet Reflection of the light

Brightness represents the intensity of light, where the higher intensity the higher brightness the given light source has. Saturation evaluates clearness of light. The higher is saturation the narrower is the spectrum of color frequencies in light. Brilliance determines the amount of achromatic element in light with a specific dominant frequency.

## Colors

The term 'color' means the attribute of an object (e.g., abscissa, text, similar). Different colors, which are used in creating an image, are composed from primary colors. Computer display boxes use a composition of red(R), green(G) and blue(B) colors. In the case of ink printers there is the set of   CMY colors . Each primary color in a RGB computer display box has an intensity value ranging from 0 to 255. Therefore, in total there are 256^3 combinations of colors. White color is received through setting RGB colors to maximum (it means: R=255, G=255, B=255). While the values of individual primary colors are equal and we are changing their intensity from 0 to 255, we get gradually 256 shades of gray color. In reproducing a color picture into a gray hue picture we cannot average the RGB components of one color, because the human eye does not perceive intensity of all colors in the same way. The highest sensitivity is to green, then to red, and least sensitive to blue color.

The overall brightness of a color can be expressed using the formula:

I = 0.299*R + 0.587*G + 0.114*B

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Applet Brightness of color perceived by human eye with RGB models

Copyright (c) 1999-2018 Juraj Štugel. www.netgraphics.sk