PRISM AND NATURE OF LIGHT:
Before Isaac Newton, it was believed that white light was colorless, and that the prism itself produced the color. Newtons 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 Newtons particle theory with Huygens wavetheory to show that color is the visible manifestation of lights 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 separated colors that are already there. He also used a lens and a second prism to recompose the spectrum back into white light.
HOW DOES A PRISM WORK:
Light changes speed as it moves from one medium to another (for example, from air into the glass of the prism). This speed change causes the light to be refracted and to enter the new medium at a different angle(Huygens principle). The degree of bending of the lights 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(Snells law). The refractive index of many materials (such as glass) varies with the wavelength or color of the light used, a phenomenon known as Dispersion. This causes light of different colors to be refracted differently and to leave the prism at different angles, creating an effect similar to a rainbow. This can be used to separate a beam of white light into its constituent spectrum of colors.
Prisms will generally disperse light over a much larger frequency bandwidth than diffraction gratings, making them useful for broad-spectrum spectroscopy. Furthermore, prisms do not suffer from complications 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, total internal occurs and all of the light is reflected. This makes a prism a useful substitute for a mirror in some situations.
Angle of Minimum Deviation:
A) Definition: The minimum value of angle of deviation, is called angle of minimum deviation. It is represented by the symbol Dm. .
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(Dm), then, i and e becomes equal.
TYPES OF PRISM:
Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet). Dispersive prisms are used to break up light into its constituent spectral colors because the refractive index depends on frequency; the white light entering the prism is a mixture of different frequencies, each of which gets bent slightly differently. Blue light is slowed down more than red light and will therefore be bent more than red light.