Lens Formula
1 / f = 1 / v – 1 / u where, f = focal length of the lens, U = distance of object, U = distance of image.
Lens Maker’ s formula 1 / f =( μ – 1)
( 1 / R1 – 1 / R2)
where, μ = refractive index of the material of the lens and R 1 and R 2 are radii of curvature of the lens.
Power of a Lens
The reciprocal of the focal length of a lens, when it is measured in metre, is called power of a lens.
Power of a lens,( P)= 1 / f( metre) Its unit is diopter( D).
The power of a convex( converging) lens is positive and for a concave( diverging) lens it is negative.
Focal Length of a Lens Combination
( i) When lenses are in contact 1 / F – 1 / f 1 + 1 / f 2 Power of the combination P = P 1 + P 2( ii) When lenses are separated by a distance d 1 / F = 1 / f 1 + 1 / f 2 – d / f 1 f 1
Power of the combination P = P 1 + P 2 – dP 1 P 2
Linear Magnification m = I / O = v / u
For a small sized object placed linearly along the principal axis, its axial( longitudinal) magnification is given by