﻿ An object of size 7.0 cm is placed at 27 cm in front of a concave
A doctor has prescribed a corrective lens of power + 1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

Here,                    P = + 1.5 D

As the focal length is positive, the prescribed lens is converging.
Spherical mirrors are of two types:
(i) Concave mirror: A spherical mirror is concave if it is silvered on the outer bulged surface and reflects light from the inner hollow surface [Fig. (b)]
.
Fig. (a) A spherical shell cut by a plane surface, (b) concave mirror, and (c) convex mirror.
(ii) Convex mirror: A spherical mirror is convex if it is silvered on the inner hollow surface and reflects light from the outer bulged surface.

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# An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed so that a sharp focussed image can be obtained? Find the size and the nature of the image.

We are given a concave mirror.

Here,
Object size, h = + 7.0 cm
Object distance, u = - 27 cm
Focal length, f = - 18 cm
Image distance, v = ?

Image size, h' = ?

Now, using the mirror formula,

$\therefore$

$⇒$

i.e.,

The screen should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image.

Magnification,

$\therefore$ Image size,

The image is real, inverted and enlarged in size.

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