Question

Find the diameter of the image of the moon formed by a spherical concave mirror of focal length 7.6m. The diameter of the moon is 3450km and the distance between the earth and the moon is 3.8 x 10 ^5km.

Answer 1

Given :

focal length=f= -7.6cm

diameter of moon= 3450km

u = - 3.8 x 10 ⁵ = which is very large as compared to f . so we can consider as ∞

1/f=1/v+1/u

1v =- 1/7.6

v= -7.6m

m = -v/ u = dimage / dobject

dimage/ 3450 x 10³ = -(7. 6 )/ - 3.8 x 10⁸ m

dimage = 7.6 x 3450 x 10³/3.8 x 10 ⁸

= 0.069 m

= 6.9 cm