Question

A lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. Applying the formula of image formation by a single spherical surface successively at the two surfaces of a thin lens, a formula known as lens maker’s formula and hence the basic lens formula can be obtained. The focal length (or power) of a lens depends on the radii of its surfaces and the refractive index of its material with respect to the surrounding medium. The refractive index of a material depends on the wavelength of light used. Combination of lenses helps us to obtain diverging or converging lenses of desired power and magnification.

The focal length of an equiconvex lens ______.

पर्याय

• increases when the lens is dipped in water.

• increases when the wavelength of incident light decreases. 

• increases with decrease in radius of curvature of its surface. 

• decreases when the lens is cut into two identical parts along its |principal axis.

Answer 1

The focal length of an equiconvex lens increases when the lens is dipped in water.

Explanation:

The focal length of an equiconvex lens rises when it is submerged in water because the refractive indices of the water and the lens material differ.

The focal length of an equiconvex lens is

`1/f = (mu - 1)(2/R)`

⇒ `f = R/(2(mu - 1)`