A ray of light passing through the optical centre of a lens will emerge without any deviation.

A person who can see the object clearly at a distance of $10 \mathrm{cm}$, requires spectacles to be able to see clearly objects at a distance of $30 \mathrm{cm}$. What type of spectacles should he use? Find the focal length of the lens.

Assertion (A): The focal length of lens does not change when red light is replaced by blue light.

Reason (R): The focal length of lens does not depend in colour of light used.

A screen is placed at a distance of $90 \mathrm{cm}$ from an object. A convex lens forms the image of this object on the screen, when placed at two different locations separated by $30 \mathrm{cm}$ as shown in the figure. Determine the focal length of the lens.

Look at the ray diagram shown, what will be the focal length of the 1^st and the 2^nd lens, if incident light ray is parallel to emergent ray.


              

A double convex lens, made of a material of refractive index ${\mu }_1$ is placed inside two liquids or refractive indices ${\mu }_2$ and ${\mu }_3$ as shown. ${\mu }_2>{\mu }_1>{\mu }_3$. A wide, parallel beam of light is incident on the lens from the left. The lens will give rise to