Question

please answer fastly...
I will mark u as a brainlist

Answer 1

We know that the lens maker's Equation for thin lenses is given as:

1/f = (n-1)(1/R1 - 1/R2).


Where,

R1 and R2 = Radii of curvature of Two Sides of the lens.

n = refractive index of the material.

f = focal length.


Now,

Given R1 = 30cm.

R2 = 60cm.

n = 1.5.


Therefore,

The focal length of double convex lens 1/f = (1.5 - 1)(1/-30 - 1/60)

                                                                        = (0.5)(-1/20)

                                                                        = -0.025

                                                                 f = -40cm.


Therefore the focal length  = -40cm.


Note: I have written -30 because for a concave lens, R1 = negative and R2 = positive.



Hope this helps!