Question

Solution

Correct option is

40 cm

For the focal length f of a lens in air, we have Where n is the refractive index of lens material (glass) with respect to air, and R1 and R2 are radii of curvature of the lens surfaces.

Here n = 1.5, R1 = +20 cm and R2 = ∞ (plane). or     f = 40 cm.

SIMILAR QUESTIONS

Q1

An illuminated object is placed between two plane mirrors mutually perpendicular to each other how many images are formed?

Q2

A 2.0 cm tall object is placed 15 cm from a concave mirror of focal length 10 cm. How far is the image from the mirror?

Q3

An object 1.0 cm tall is placed 8 cm in front of a concave mirror of radius of curvature 24 cm. What is the size and the nature of the image?

Q4

A concave mirror forms a real image four times the size of the object placed at a distance of 10 cm from it. What is the radius of curvature of the mirror?

Q5

In Young’s double slit experiment, the 10th maximum of wavelength λ1 is at a distance y1 from its central maximum and the 5th maximum of wavelength λ2 is at a distance y2 from its central maximum. The ratio y1/y2will be

Q6

White light is used to illuminate the two slits in Young’s double slit experiment. The separation between the slits is d and the distance between the screen and the slit is D (>> d). At a point on the screen directly in front of on the slits, certain wavelengths are missing. The missing wavelengths are (here m = 0, 1, 2, … is an integer)

Q7

In Young’s double slit experiment using two identical slits, the intensity of the maximum at the centre of the screen is I. What will be the intensity at the centre of the screen if one of the slits is closed?

Q8

Glycerine (refractive index 1.4) is poured into a large jar of radius 0.2 m to a depth of 0.1 m. There is a small light source at the centre of the bottom of the jar. Find the area of the surface of glycerine through which the light passes.

Q9

A ray of light travelling in glass (refractive index, ang = 3/2) is incident on a horizontal glass-air surface at the critical angle c. If a thin layer of water (refractive index, anw = 4/3) is now poured on the glass-air surface, at what angle will the ray of light emerge into air at the water-air surface?

Q10

The thickness of a plano-convex lens is 4 cm. When it is placed on a horizontal table in such a way that its curved surface be in contact with the table, then the depth of a bottom-point of the lens appears 3 cm. If the lens is inverted so that its plane surface be in contact with the table, then the apparent depth of the centre of the lens-surface is found to be (25/8) cm. Determine the focal-length of the lens.