# (a) Define the term power of a lens. Write its SI unit.  (b) Derive the expression for the power of two thin lenses placed coaxially in contact with each other.

a) Power of a lens :

$\Rightarrow$ Power of a lens is a measure of the convergence or divergence which a lens introduces in the light falling on it.

$\Rightarrow$ It is defined as the tangent of the angle by which it converges or diverges a beam of light falling at unit distance from the optical centre.

$\Rightarrow$SI unit of power of a lens is dioptre (D).

$1D=1m^{-1}$

b) The two lens - the system is regarded as equivalent to a single lens of focal length f,

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$

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