# Prism and Lens

The light travelling from one medium to another of different optical densities gets refracted. Light of all wavelengths travels with the same speed in air or vacuum. When the light rays travel from one medium to another, there is a change in the speed of light. When there is a change in the speed of the light, the light gets refracted at different angles.

## Prism

In optics, a prism is a piece of glass or other transparent material that is cut with precise angles and plane faces used for analysing light. The commonly used prism has three rectangular surfaces and two triangular faces parallel to each other. The edge common to them is called the refracting angle. In a prism, the light of different wavelengths travels at different speeds in a material medium. As a result, the refractive index of a material medium is different for different wavelengths of light.

Sir Issac Newton used a triangular prism to explain the theory of light. He placed a prism near the window through which sunlight was entering the room; this caused the full spectrum of light to be seen on the opposite wall. This is called dispersion. A medium in which the speed of light varies with wavelength is called the dispersive medium. A spectrum in which the different colours of light are distinctly seen without any overlapping is called a pure spectrum.

Later, Newton placed the prism inverted in front of the spectrum of the light; this caused the coloured spectrum to come back together and become white light.

## Lens

A lens is an optical medium bounded by two surfaces, of which at least one is spherical. Spherical lenses have either two spherical surfaces or one plane, and one spherical surface Lens may be cylindrical also. The spherical lens can be concave or convex. A convex lens is thicker at the centre than at the edges, and generally, it converges a beam of light. A lens that is thinner at the centre than at the edges and diverges the beam of light is called a concave lens.

### Principal axis:

The straight line passing through the centres of curvature of the two spherical surfaces of the lens is called the principal axis of the lens.

### Optical centre:

The optical centre of a lens is that fixed point on the principal axis inside the lens such that all rays passing through it have emergent rays parallel to the corresponding incident rays.

## Power of lens

The power of a lens is its ability to converge or diverge the incident beam of light and is defined as the reciprocal of its focal length. Dioptre is the unit of power. The power of the lens is said to be one dioptre if the focal length is one metre.

The power of a convex lens is positive, while that of a concave lens is negative. Lenses are found in a huge array of optical instruments, ranging from simple magnifying glass to the eye to a camera’s zoom lens.

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