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

a needle placed 5 cm from a lens forms an image on a screen placed 90

1.	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» mv= 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= 19/90so, f= 90/19 cm= 4.736 cm@skb