Question
Obtain the magnification by lens after giving the definition.
✓
Answer
→The ratio of the size of the image obtained by lens to that of the object is called magnification.
$m=\frac{h^{\prime}}{h}......(1)$
→As shown in figure AB is an object and its image is $A ^{\prime} B ^{\prime}$.
→From figure $\triangle ABP$ and $\triangle A ^{\prime} B ^{\prime} P$ are similar triangles.
$\therefore \quad \frac{ A ^{\prime} B ^{\prime}}{ AB }=\frac{ B ^{\prime} P }{ BP }$
→$\begin{array}{lll}\text { But, } & A ^{\prime} B ^{\prime}=-h^{\prime} & AB =h \\ & B^{\prime} P =v & BP =-u\end{array}$
→Substituting these values in above equation,
$\begin{array}{l}\therefore \frac{-h^{\prime}}{h}=-\frac{v}{u} \\\therefore \frac{h^{\prime}}{h}=\frac{v}{u}......(2)\end{array}$
→Comparing equation (1) and (2),
$\therefore m=\frac{v}{u}$
→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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