Derive an experiment to determine the focal length of concave lens using a plane mirror, a converging lens and an illuminated object.

Derive an experiment to determine the focal length of concave lens using a plane mirror, a converging lens and an illuminated object. 

• A plane mirror is placed behind a concave and a convex lenses in contact
• A screen containing illuminated cross wire in a hole is moved to and fro in front of the lenses until the image of the cross wires is formed besides the object.
• The combined focal length, f is equal to half the distance between the center of the lenses and the screen.

Calculation

Let focal length of concave and convex lenses be f₁ and f₂ respectively

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