A needle of height 5 cm placed 45 cm from a lens forms an image on a screen placed 90 cm on the other side of the lens the type of lens and its focal length are

Options

A. convex 30 cm
B. concave, 30 cm
C. virtual 20 cm
D. concave 60 cm

Answer

A

Explanation

Real image is obtained on screen, hence lens is convex.
`u=-45cm,v=90cm`
`(1)/(f)=(1)/(v)-(1)/(u)=(1)/(90)-(1)/(-45)=(1+2)/(90)`
`f=30cm`