## Question

### Solution

Correct option is

90 cm, 8.2 cm

(i) If F is the focal length of the combination, then

Here f1 = f2 = +25 cm (both the lenses are convex). Therefore,

If the image is at a distance v from the combination of lenses, then

Here u = –15 cm, F = +12.5 cm.

The object is at a distance of 15 cm on one side and the image is at a distance of 75 cm on the other side of the lens-combination. Hence the distance between the object and the image is

15 cm + 75 cm = 90 cm.

(ii) When both the lenses are concave (f1 = f2 = –25 cm), then F = – 12.5 cm. Again, by the lens formula

Now the image is on the same side of the lens as the object. Hence the distance between the object and the image is

15 cm – 6.8 cm = 8.2 cm.

#### SIMILAR QUESTIONS

Q1

The radius of curvature of the convex surface of a thin plano-convex lens is 20 cm. Refractive index of the medium of the lens is 1.5. Calculate the focal length of the lens.

Q2

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.

Q3

The focal length of a convex lens in air is 20 cm. What will be its focal length if it is immersed in a liquid of refractive index 1.35? Refractive index of glass is 1.50.

Q4

A lens made of glass having refractive index 1.5 has a focal length of 50 cm in air. What will be its focal length if it is immersed in a liquid of refractive index 1.2?

Q5

A convex lens of glass (refractive index 1.5) has both surfaces of radius of curvature 20 cm. Find its focal length and nature when it immersed in a liquid of refractive index 1.75.

Q6

The focal length of a lens of glass (n = 1.5) in air is 0.4 m and in a liquid is 1.2m. Determine the refractive index of the liquid.

Q7

The radius of curvature of both the surfaces of a double convex lens is R. The refractive index of the material of the lens is 1.5. When a point-source of light is placed on the principal axis of the lens at a distance of 20 cm in-front of the one surface of the lens, then rays emerge from the other surface normally. Determine the radius of curvature R of each surface of the lens.

Q8

An air passenger sitting in an aeroplane flying at a height of 2000 meter from the earth takes photographs of the earth. The focal-length of his camera-lens is 50 cm. The size of the camera-film is (18 × 18)cm2. What maximum area can be covered by the camera?

Q9

The diameter of sun is 1.4 × 109 meter. Determine the diameter of the image of the sun formed by a convex-lens of focal length 1.0 meter. Distance of the sun from the earth is 1.0 × 1011 meter.

Q10

The image of an object is formed at a distance of 40 cm from the object, when the lens is placed exactly mid-way between the object and the image. What is the power of the lens?