Q. 3 D4.3( 3 Votes )

Obtain the lens formula for spherical lens.

Consider the image formation by convex lens as shown in the figure below.

By Cartesian sign convention;

Object distance (OB) = - u;

Image distance (OB’) = + v;

Focal length (OF2 = OF2) = f

From the geometry we can see that in the figure above right triangle ΔABO and ΔA’B’O’ are similar.

…… (1)

Also, in figure above by geometry right triangle ΔODF2 and ΔBAF2 are similar.

( OD = AB, opposite sides of rectangle ABOD are equal).

………. (2)

From (1) and (2)

.

Dividing each term by uvf, we get.

This equation is called the ‘Lens Formula’

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