Lighting Behavior

Lighting Behavior:

- Reflection
-  Absorption 
- Transmission
-  Refraction
- Interference




Reflection

Whenever light strikes a surface, three possibilities are open: it is reflected, absorbed or transmitted. Often a combination of two or even all three effects occurs. The amount of
reflected light depends on the type of surface, angle of incidence and spectral composition of the light. Reflection ranges from less than a few percent for very dark surfaces like black velvet, to over 90% for bright surfaces such as white paint. The way the light is reflected also depends on the smoothness of the surface. Rough surfaces diffuse the light by reflecting it in every direction. In contrast, smooth surfaces like the surface of still water or polished glass reflect the light back undiffused, making the surface act as a mirror. A ray of light striking a mirrored surface at an angle to the perpendicular will be reflected back at the same angle on the other side of the perpendicular (in the same way as a non-spinning billiard ball rebounds from the cushion) this is the well-known law of reflection that is given as:

angle of incidence = angle of reflection

Mirrored surfaces are very good for directing light beams to where we want them. Curved mirror reflectors are widely used for focusing light, dispersing it or creating parallel or divergent beams, and are all governed by the law of reflection.

Absorption 

 If the material’s surface is not entirely reflecting or the material is not a perfect transmitter, part of the light will be absorbed. It ‘disappears’ and is basically converted into heat. The percentage of light absorbed by a surface (i.e. absorbance) depends on both the angle of incidence, and on the wavelength. The absorption of light makes an object dark to the wavelength of the incoming radiation. Wood is opaque to visible light. Some materials are opaque to some frequencies of light, but transparent to others. Glass is opaque to ultraviolet radiation below a certain wavelength, but transparent to visible light.

 Transmission

Transparent materials transmit some of the light striking its surface, and the percentage of light that is transmitted is known as its transmittance. High transmittance materials such as clear water and glass transmit nearly all the light that’s not reflected. Low transmittance materials, such as paper, transmit only a small percentage of this light.

Refraction
If a light ray passes from one medium into another of different optical density (and at an
angle other than perpendicular to the surface between the two media), the ray will be ‘broken’. This behavior is called refraction, and is caused by the change of speed of the light as it passes between transparent media of different optical densities. 

Interference 

The wave nature of light also leads to the interesting property of interference. A familiar example of this is when there is a thin film of oil floating on the surface of a pool. Sometimes the oil will display a brilliant pattern of colors or rainbows, even when illuminated by white light. The irising colors of the Peacock’s tail feathers are caused by interference of light and not by pigments. What is happening is that different parts of the oil film cause the different wavelengths in the white light to interfere and produce different wavelengths (=colors)various colors are generated, depending on the thickness of the fil where the interference occurs. Similar examples of interference are found when looking at soap bubbles, or at the surface of a CD.