Question

A concave mirror forms an image of 20cmhigh object on a screen placed 5.0m away from the mirror. The height of the image is 50cm. Find the focal length of the mirror and the distance between the mirror and the object.

Answer 1

Given :

ho=20cm

image distance= v= - 5m=- 500cm

hi= -50cm

u=?

magnification = m= -v/u= hi/ho

m= -50/20= -5/2

m=-v/u

u=- v /m

= - 500x 2/5

= - 200 cm = - 2m

Focal length=f?

from mirror equation :

1/f=1/v+1/u

= -1/5 - 1/2

1/f= -2-5/10

1/f= - 7/10

f= -10/7 = - 1.44 m

∴ Focal length = f= - 1.44m

Answer 2

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