• question_answer (a) Define optical centre of a spherical lens. (b) A divergent lens has a focal length of 20 cm. At what distance should an object of height 4 cm from the optical centre of the lens be placed so that its image is formed 10 cm away from the lens. Find the size of the image also. (c) Draw a ray diagram to show the formation of image in above situation.

 (a) Optical centre is a point on a lens through which ray of light passes undeviated, without suffering any refraction. (b) Given: $f=\text{ }20\text{ }cm,\text{ }{{h}_{0}}=4\,cm,\text{ }v=\,\,10\,cm$ We know that, $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$ $\frac{1}{-20}=-\frac{1}{10}-\frac{1}{u}$ $\frac{1}{u}=-\frac{1}{10}+\frac{1}{20}$ $\frac{1}{u}=-\frac{-2+1}{20}=\frac{-1}{20}$ $u=-20cm$ Now,     $m=\frac{hi}{ho}=\frac{v}{u}$ $\frac{hi}{4}=\frac{-10}{-20}$ $2{{h}_{i}}=4$ $\Rightarrow$               ${{h}_{i}}=2\mathrm{cm}$