Question

An object 3 cm high produces a real image 4-5 cm high, when placed at a distance of 20 cm
from a concave mirror. Calculate : (i) the position of image (ii
) focal length of the concave
mirror.
​

Answer 1

Answer:-

Here,

object height, h1 = 3 cm

image height, h2 = 4.5 cm

we know that,

m = h2 / h1 = - v / u

So,

4.5/3=-v /-20 (object distance =20)

4.5 (-20/3) = - v

v= 30 cm

So, image is 30 cm from the mirror.

Now , using mirror formula

1/u+1/v=1/f

1/-20+1/-30=1/f

-5/60=1/f

-1/12=1/f

f=-12

Hence , the required focal length is 12 cm.

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Answer 2

Answer:

image height =~5cm

image height/object height= -v/u

according to sign convention

v= 33.3cm