Q5 of 13 Page 127

Solve the following examples

An object of height 7 cm is kept at a distance of 25 cm in front of a concave mirror. The focal length of the mirror is 15 cm. At what distance from the mirror should a screen be kept so as to get a clear image? What will be the size and nature of the image?


According to the question,

Object distance (u) = -25 cm;


Height of object (Ho) = + 7 cm;


Focal length (f) = -15 cm;


Image distance = v cm;


By Mirror Formula:



1/v + 1/u = 1/f.


Putting values we get,



⇒


⇒


Taking the lcm, we get,


⇒


⇒


⇒


⇒ v = -37.5 cm


∴ the image distance = 37.5 cm (The negative sign indicates that the image is formed on the left side of the mirror)


Now, image distance (v) = -37.5 cm


Magnification =


Hi is the image height.


HO is the object height.


“v” is the image distance


“u” is the object distance


⇒


⇒


⇒ Hi = -10.5 cm.


Size of image is 10.5 cm which is negative hence image is real .


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3

Draw ray diagrams for the cases of images obtained in concave mirrors as described in the table on page 122.

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Solve the following examples

A convex mirror has a focal length of 18 cm. The image of an object kept in front of the mirror is half the height of the object. What is the distance of the object from the mirror?

 5

Solve the following examples

A 10 cm long stick is kept in front of a concave mirror having focal length of 10 cm in such a way that the end of the stick closest to the pole is at a distance of 20 cm. What will be the length of the image?