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INTRODUCTION:
In optics,a
prism
is a transparent optical element with flat, polished surfaces that refract light.The exact angles between the surfaces depend on the application. The traditionalgeometrical shape is that of a triangular prism with a triangular base and rectangular sides,and in colloquial use "prism" usually refers to this type. Some types of optical prism are notin fact in the shape of geometric prisms.Prisms can be made from any material that istransparent to the wavelengths for which they are designed. Typical materials include glass, plastic and fluorite. A prism can be used to break light up into its constituent spectral colors (the colors of therainbow). Prisms can also be used to reflect light, or to split light into components withdifferent polarizations.


PRISM AND NATURE OF LIGHT:
Before Isaac Newton,it was believed that white light was colorless, and that the prism itself produced the color. Newton's experiments demonstrated that all the colors already existed inthe light in a heterogeneous fashion, and that "corpuscles" (particles) of light were fanned out because particles with different colors traveled with different speeds through the prism. It wasonly later that Young and Fresnel combined Newton's particle theory with Huygens' wavetheory to show that color is the visible manifestation of light's wavelength. Newton arrived at his conclusion by passing the red color from one prism through a second prism and found the color unchanged. From this, he concluded that the colors must already be present in the incoming light
—
thus, the prism did not create colors, but merely separatedcolors that are already there. He also used a lens and a second prism to recompose thespectrum back into white light. This experiment has become a classic example of themethodology introduced during the scientific revolution.The results of this experimentdramatically transformed the field of metaphysics,leading to John Locke's primary vssecondary quality distinction. Newton discussed prism dispersion in great detail in his book
Opticks
.
[1]
He also introducedthe use of more than one prism to control dispersion.
[2]
Newton's description of hisexperiments on prism dispersion was qualitative, and is quite readable. A quantitative wereintroduced in the 1980s.
[3]

THEORY:
HOW DOES A PRISM WORK:
Light changes speed as it moves from one medium to another (for example, from air into theglass of the prism). This speed change causes the light to be refracted and to enter the newmedium at a different angle(Huygens principle). The degree of bending of the light's path
depends on the angle that the incident beam of light makes with the surface, and on the ratio between the refractive indices of the two media(Snell's law). The refractive index of manymaterials (such as glass) varies with the wavelength or color of the light used, a phenomenonknown as
dispersion
.This causes light of different colors to be refracted differently and toleave the prism at different angles, creating an effect similar to a rainbow.This can be used toseparate a beam of white light into its constituent spectrum of colors. Prisms will generallydisperse light over a much larger frequency bandwidth than diffraction gratings,making themuseful for broad-spectrum spectroscopy.Furthermore, prisms do not suffer fromcomplications arising from overlapping spectral orders, which all gratings have.Prisms are sometimes used for the internal reflection at the surfaces rather than for dispersion. If light inside the prism hits one of the surfaces at a sufficiently steep angle, totalinternal reflection occurs and
all of the light is reflected. This makes a prism a usefulsubstitute for a mirror in some situations.

Angle of Minimum Deviation:
A)

Defination:The minimum value of angle of deviation, is called angle of minimumdeviation. It is represented by the symbol D
m
.
b) Explanation:For same angle of deviation (D) there are two values of angle of
incidence. One value equals ’i’ and other value equals ‘e’

As angle ‘i’ is increased from a small value, ‘e’ decreases from large value and
angle of deviation decreases. When angle of deviation is minimum(D
m
), then, ‘i’ and ‘e’ becomes equal.