When blue light is used instead of red light in a convex lens,its focal length will ......... .

Refractive index of a glass convex lens is $1.5$. The radius of curvature of each of the two surfaces of the lens is $40 \text{ cm}$. The ratio of the power of the lens when immersed in a liquid of refractive index $1.25$ to that when placed in air is

$A$ convex lens is made up of three different materials as shown in the figure. For a point object placed on its axis,the number of images formed are

$A$ double convex lens,made of a material of refractive index $\mu_1$,is placed inside two liquids of refractive indices $\mu_2$ and $\mu_3$ as shown,where $\mu_2 > \mu_1 > \mu_3$. $A$ wide,parallel beam of light is incident on the lens from the left. The lens will give rise to:

$A$ screen is placed $90\, cm$ from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by $20\, cm$. The focal length of the lens is.....$ cm$

What are paraxial rays?