# A Combination of Lenses for a Camera Contains Two Converging Lenses of Focal Lengths 20 Cm and 40 Cm and a Diverging Lens of Focal Length 50 Cm. Find the Power and Focal Length of the Combination.A Optical Instrument in Which the Above Arrangement of Convex Lens is Used is a Convex Lens Used to Burn Paper by Focusing Sunlight. - Science

A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.A optical instrument in which the above arrangement of convex lens is used is a convex lens used to burn paper by focusing sunlight.

#### Solution

Focal length (f1) of the 1st converging lens = 20 cm = 0.2 m
Focal length (f2) of the 2nd converging lens = 40 cm = 0.4 m
Focal length (f3) of the diverging lens = -50 cm = -0.5 m (negative sign due to sign convention)

Power  = 1/"foccal length of lens (in metre)"

Power (P1) of 1st lens = 1/0.2=2.5D

power (P2) of 2nd lens = 1/0.4=2.5 D

Power (P3) of diverging  lens =1/0.5 =-2D

Net power of the combination = P1 + P2P3 = 5 + 2.5 + (-2) = 5.5 D

Net focal length of the combination = 1/"net power"=1/5.5=0.1818m=18.18 cm

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#### APPEARS IN

Lakhmir Singh Class 10 Physics (Science)
Chapter 5 Refraction of Light
Q 29.3 | Page 262