Question

an object 1 metre tall is placed on the principal axis of a convex lens and its 40cm images formed on the screen please as a distance of 70 cm from the object find the focal length of the lens​

Answer 1

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Given:

h= 1m = 100cm

h'= 40cm

u+v= 70cm

let u = -x

v= 70-x

Focal length, f= ?

Magnification, m = h'/h= v/u

m= -40/100= (70-x)/(-x)

40x= 7000-100x

x= 50cm

u= -x = -50cm

v= 70-x = 70-50= 20cm

Substituting the values of u and v in the lens formula,

1/v - 1/u = 1/f

1/20 - 1/(-50) =1/f

1/20 + 1/50= 1/f

70/1000= 1/f

f= 100/7

f= 14.3cm

Answer 2

Answer:

above one is your answer.

Explanation:

hope it helps you