In the following diagram, an object ‘O’ is placed 15 cm in front of a convex lens L1 of focal length 20 cm and the final image is formed at ‘I’ at a distance of 80 cm from the second lens L₂. Find the focal length of the lens L₂.

Question

Philoid · Accepted Answer

Given: Object distance, u = 15 cm

Focal length of first lens, f₁= 20 cm

Image distance for second lens, v₁=80 cm

Separation between two lenses, d=20 cm

Calculation:

Using lens maker’s formula, we have,

The image of lens L₁ will be acting as object for lens L₂.

u₁=-60+20=-40

Using lensmaker’s formula on lens 2 we get,

The focal length of the second lens is 26.67 cm.

In the following diagram, an object ‘O’ is placed 15 cm in front of a convex lens L1 of focal length 20 cm and the final image is formed at ‘I’ at a distance of 80 cm from the second lens L2. Find the focal length of the lens L2.