Light

Light:
Light is electromagnetic radiation of a wavelength that is visible to the human eye (in a range from about 380 or 400 nanometers to about 760 or 780 nm).In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.

Electromagnetic spectrum:
Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.

The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

Light sources
There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. Examples include sunlight , incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". Blue thermal emission is not often seen. The commonly seen blue colour in a gas flame or a welder's torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm).

Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.

Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.

Reflection:
The first property of light we consider is reflection from a surface, such as that of a mirror.


When light is reflected off any surface, the angle of incidence is always equal to the angle of reflection .
Note:
The angles are always measured relative to the normal to the surface.
The law of reflection is also consistent with the particle picture of light.
Refraction

Refraction is the bending of light as it passes between materials of different density.
The index of refraction of a material is the ratio of the speed of light in vacuum to the speed of light in that material:
n =c/v
where v is the speed of light in the material.

Note:
The more dense the material, the slower the speed of light in that material. Thus
The frequency of light does not change when it passes from one medium to another. but the wavelength must change. The index of refraction can therefore be written in terms of wavelengths as:
n =0/
where 0 is the wavelength of the light in the vacuum and  is the wavelength of the light in the medium.




Explanation for Refraction of Light
The change in speed and wavelength at the boundary between two materials causes light to change direction. Think of a car approaching a patch of mud at a sharp angle from a well paved road. The tire that hits the mud first will slow down, while the other tire is still going fast on the good road. This will cause the car to turn, until both tires are in the mud and going at the same speed.



Refraction between two substances, n1 being more dense.
Note:
This relationship between the angles is called Snell's Law.
The relation between the two angles is the same whether the ray is moving from medium 1 to 2 (so that O1 is the angle of incidence and O2 is the angle of refraction) or whether the ray moves from medium 2 to medium 1, so that O2 is the angle of incidence and O1 is the angle of refraction.

Total Internal Reflection
For a light ray passing from a more dense to a less dense material, there is a critical angle of incidence qc for which the angle of refraction is 90°. For greater angles of incidence, the light cannot pass through the boundary between the materials, and is reflected within the more dense material.