Question
Obtain the magnification by lens after giving the definition.
✓
Answer
$\rightarrow$ The ratio of the size of the image obtained by lens to that of the object is called magnification.
$m=\frac{h^{\prime}}{h}......(1)$
$\rightarrow$ As shown in figure $AB$ is an object and its image is $A ^{\prime} B ^{\prime}$.
$\rightarrow$ From figure $\triangle ABP$ and $\triangle A ^{\prime} B ^{\prime} P$ are similar triangles.
$\therefore \frac{ A ^{\prime} B ^{\prime}}{ AB }=\frac{ B ^{\prime} P }{ BP }$
$\rightarrow$ But $, A ^{\prime} B ^{\prime}=-h^{\prime} AB =h$
$ B^{\prime} P =v BP =-u$
$\rightarrow$ Substituting these values in above equation,
$\therefore \frac{-h^{\prime}}{h}=-\frac{v}{u}$
$\therefore \frac{h^{\prime}}{h}=\frac{v}{u}......(2)$
$\rightarrow$ Comparing equation $(1) $ and $(2),$
$\therefore m=\frac{v}{u}$
$\rightarrow$ If the image formed by lens is virtual then magnification $(m)$ is positive and if the image formed by lens is real then magnification $(m)$ is negative.
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