Page 147 - NCERT Science Class 10 English Medium

n Try with plane mirrors of different sizes. Did you see the entire
object in the image?
n Repeat this Activity with a concave mirror. Did the mirror show
full length image of the object?
n Now try using a convex mirror. Did you succeed? Explain your
observations with reason.

You can see a full-length image of a tall building/tree in a small
convex mirror. One such mirror is fitted in a wall of Agra Fort facing Taj
Mahal. If you visit the Agra Fort, try to observe the full image of Taj
Mahal. To view distinctly, you should stand suitably at the terrace
adjoining the wall.
Uses of convex mirrors
Convex mirrors are commonly used as rear-view (wing) mirrors in
vehicles. These mirrors are fitted on the sides of the vehicle, enabling the
driver to see traffic behind him/her to facilitate safe driving. Convex
mirrors are preferred because they always give an erect, though
diminished, image. Also, they have a wider field of view as they are curved
outwards. Thus, convex mirrors enable the driver to view much larger
area than would be possible with a plane mirror.

Q U E S T I O N S

1. Define the principal focus of a concave mirror.
2. The radius of curvature of a spherical mirror is 20 cm. What is its focal
length? ?

3. Name a mirror that can give an erect and enlarged image of an object.
4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

9.2.3 Sign Convention for Reflection by Spherical Mirrors

While dealing with the reflection of light by spherical mirrors, we shall
follow a set of sign conventions called the New Cartesian Sign
Convention. In this convention, the pole (P) of the mirror is taken as the
origin (Fig. 9.9). The principal axis of the mirror is taken as the x-axis
(X’X) of the coordinate system. The conventions are as follows –
(i) The object is always placed to the left of the mirror. This implies
that the light from the object falls on the mirror from the left-hand
side.
(ii) All distances parallel to the principal axis are measured from the
pole of the mirror.
(iii) All the distances measured to the right of the origin (along
+ x-axis) are taken as positive while those measured to the left of
the origin (along – x-axis) are taken as negative.
(iv) Distances measured perpendicular to and above the principal axis
(along + y-axis) are taken as positive.
(v) Distances measured perpendicular to and below the principal axis
(along –y-axis) are taken as negative.

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