Question

a needle placed 5 cm from a lens forms an image on a screen placed 90 cm on either side of lens determine the focal length of convex lens

Answer 1

Here is your answer

here,
u= -5 cm
v= 90 cm (since image is formed on screen of convex lens so it is real)

by lens formula
1/f = 1/v - 1/u
= 1/90 -( 1/-5)
= 1/90+ 1/5
= (1+18)/90
= 19/90

so, f= 90/19 cm
= 4.736 cm


HOPE IT IS USEFUL

Answer 2

u= -5 cm

v= 90 cm (since image is formed on screen of convex lens so it is real)

by lens formula

1/f = 1/v - 1/u

= 1/90 -( 1/-5)

= 1/90+ 1/5

= (1+18)/90

= 19/90

so, f= 90/19 cm

= 4.736 cm

@skb