Question

In the given figure a plano - concave lens is placed on a paper on which a flower is drawn. How far above its actual position does the flower appear to be ?

• A. $10\text{cm}$
• B. $15\text{cm}$
• C. $50\text{cm}$
• D. None of these

Answer 1

The correct option is A $10\text{cm}$


Considering refraction at the curved surface.
$u=-20;{\mu }_2=1$
${\mu }_1=\frac{3}{2};R=+20$
applying $\frac{{\mu }_2}{v}-\frac{{\mu }_1}{u}=\frac{{\mu }_2-{\mu }_1}{R}\Rightarrow \frac{1}{v}-\frac{\frac{3}{2}}{-20}=\frac{1-\frac{3}{2}}{20}\Rightarrow v=-10$
i.e $10\text{cm}$ below the curved surface or $10\text{cm}$ above the actual position of flower.