Lenses and their parts & rules to draw ray diagrams for lenses

LENSES:

A lens is formed when a transparent material is bounded by two surfaces

of which one (or) both surfaces are spherical. That is a lens is bounded by

at least one curved surface.

Symbolically convex and concave lenses are represented as follows

CONVEX LENS IS ALSO CALLED AS CONVERGING LENS AND CONCAVE LENS IS ALSO CALLED AS DIVERGING LENS

PARTS OF LENSES:

In case of bi convex and bi concave lenses we have two foci and two centre of curvatures, as we get two spheres on either side, if we extend the curvatures with imaginary lines. Observe the figures below.

In the above figures F1,C1 belongs to first surface and F2,C2 belongs to second surface.

OPTIC CENTRE (P):

The midpoint on the lens is called optic centre.

CENTRE OF CURVATURE (C): The centre of the spherical surface of a lens is called centre of curvature.

PRINCIPAL AXIS:

The line passing through optic centre and centre of curvature is called principal axis.

PRINCIPAL FOCUS (F):

A Point on the principal axis where all the rays parallel to principal axis after reflection converge (in case of convex lens) to or appears to diverge from (in case of concave lens).

RADIUS OF CURVATURE (R):

The distance from centre of curvature to optic centre is called Radius of curvature.

FOCAL LENGTH (f):

The distance from focus to optic centre is called focal length.

RULES TO DRAW RAY DIAGRAMS FOR LENSES:

Rule 1:

the ray passing parallel to principal axis, after refraction converges to focus or appears to be diverging from focus.

Rule 2:

the ray passing through focus or appears to be passing through focus, after refraction becomes parallel to principal axis.

Rule 3:

the ray passing through optic centre is un deviated (continues in the same path).

Rule 4:

the ray passing through principal axis is un deviated as this ray also passes through optic centre.

Like reflection, here also we make use of any two rules to show the formation of image. So, the minimum number of rays required to show the formation of image are 2.

(HINT: if you have problem in understanding the rules, it’s better to choose rule3 as one of the two rules to be used. Because it’s very easy to apply)

thank you

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