Question

An illuminated object and a screen are placed 90cm apart. What is its focal length and nature of the lens required to produce a real image on the screen twice the size of the object.

Answer 1

Let object be at distance u and image at v from lens. We can use ony convex lens as the image is real and inverted.

Thus v = 90-u

let the focal length be f

and since magnification is m = 2

we can use the relation that:

v/u = m = 2

thus v = 2u

hence, v= 60 cm

and u = -30 cm