Describe an experiment to determine the focal length of thin converging lens mounted inside a short cylindrical tube

Describe an experiment to determine the focal length of thin converging lens mounted inside a short cylindrical tube

• The tube is placed between an illuminated object and a screen a distance l slightly more than 4 times the approximate focal length of a lens to form a clear magnified image at the screen. The position of the front part P₁ is noted
• The tube is moved towards the screen until a clear diminished image is formed on the screen and position P₂ of the front part is noted
• 

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