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