Question

At what distance should an object be placed in concave mirror with focal distance 15cm .Show that a real image of 5times of the size of object may form .Find the position of image also.

Answer 1

According to the question, we know that magnification of the image is 5. Since the mirror is concave, so image will form below the principle axis.

Using sign convention, the magnification will be "-5".

now, we know:

m = 5

f = 15 cm

m = -v/u

-5 = -v/u

5u = v------------( 1 )

1/f = 1/v + 1/u

1/v = 1/f - 1/u

1/v = u - 15/15u

therefore, v = 15u/ u - 15----------------( 2 )

Putting the value of ( 2 ) into ( 1 )

5u = 15u/ u - 15

Therefore,

u - 15 = 3

u = 18 cm.

Using ( 1 )

5u = v

v = 5u

v = 5 * 18

Therefore,

v = 90 cm.