Question

A compound microscope consists of two converging lenses. One of them, of smaller aperture and smaller focal length, is called objective and the other of slightly larger aperture and slightly larger focal length is called eye-piece. Both lenses are fitted in a tube with an arrangement to vary the distance between them. A tiny object is placed in front of the objective at a distance slightly greater than its focal length. The objective produces the image of the object which acts as an object for the eye-piece. The eye-piece, in turn, produces the final magnified image.

The focal lengths of the objective and eye-piece of a compound microscope are 1.2 cm and 3.0 cm respectively. The object is placed at a distance of 1.25 cm from the objective. If the final image is formed at infinity, the magnifying power of the microscope would be:

Options

• 100

• 150

• 200

• 250

Answer 1

200

Explanation:

Applying the lens formula for the objective lens,

`1/"v"_0 - 1/"u"_0 = 1/"f"_0`

Or, `1/"v"_0 + 1/1.25 = 1/1.2`

∴ v₀ = 30 cm

Magnification = `"v"_0/"u"_0 "D"/"f"_"e"`

= `30/1.25 xx 25/3`

= 200