Question

An object O is placed at a distance of 15 cm from a convex lens A of focal length 10 cm and its image I₁ is formed on a screen on the other side of the lens. A concave lens B is now placed midway between A and I₁ , then the screen is moved back 10 cm to receive a clear image I₂ . 

Find (a) The focal length of the concave lens and 

(b) Linear magnification of the final image.

Answer 1

The convex lens forms the image of the object at point I₁ . This image acts as the object for concave lens and the final image is formed at I₁’. 

For refraction through convex lens,

The negative sign implies it is a concave lens.

Linear magnification 

The negative sign shows that the final image is inverted.