(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. 

 

 

 

 
 
 
 
 

Answers (1)
S safeer

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.

\RightarrowSI 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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