Refraction through a lens

We can place the objects at Infinity. This is shown for two incident rays on the diagram below.

refraction by lens derivation

The power P of a lens is defined as the inverse of its focal length. Focal length of a convex lens: The distance between the optical centre and principal focus of a lens is called focal length.

At this boundary, the light ray is passing from air into a more dense medium usually plastic or glass.

Refraction through a lens

Yet, there are three specific rays that behave in a very predictable manner. This can also be shown through an experiment. The net effect of the refraction of light at these two boundaries is that the light ray has changed directions.

converging lens

These specific rays will exit the lens traveling parallel to the principal axis. The more powerful the lens, the closer to the lens the rays will cross. A lens has two principal foci.

Refraction of light

The extension of the refracted rays will intersect at a point. A concave lens bulges inward and is thinner in the middle and thicker at the edges. For such thin lenses, the path of the light through the lens itself contributes very little to the overall change in the direction of the light rays. Another ray of light passes through optical centre of the lens and goes straight. Rules for obtaining images formed by convex lens In convex lens, the image is always formed at a point where at least two refracted light rays meet. Since the light ray is passing from a medium in which it travels fast less optically dense into a medium in which it travels relatively slow more optically dense , it will bend towards the normal line. The light rays are refracted after passing through the lens. The ray of light passing through optical centre goes straight and does not deviate. Rule 2: A ray of light passing through the optical centre of the concave lens goes straight after refraction through the lens. If the light rays are coming from left hand side they will converge at right hand side of the lens and vice versa.
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Refraction by spherical lens and image formation by lens