Two lenses of focal lengths 20 cm and – 40 cm are held in contact. The image of an object at infinity will be formed by the combination at (a) 10 cm (b) 20 cm (c) 40 cm (d) infinity

Two lenses of focal lengths 20 cm and – 40 cm are held in contact. T

1.	Two lenses of focal lengths 20 cm and – 40 cm are held in contact. The image of an object at infinity will be formed by the combination at (a) 10 cm (b) 20 cm (c) 40 cm (d) infinity
Answer» The image is formed at the focus, f = 10 cmExplanation:f1 = f2 = 10 cmFocal length of lenses in COMBINATION : 1 /f = 1/f1 + 1 /f21 /f = 1 /20 + 1 /20 = 2 /20f = 10 cmu = infinityLens formulae : 1/f = 1 /v - 1 /u1 /v = 1 /f + 1/u ( 1/∞ = 0 ) = 1 /10 + 0 = 1 /10V = 10 CM

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Two lenses of focal lengths 20 cm and – 40 cm are held in contact. The image of an object at infinity will be formed by the combination at (a) 10 cm (b) 20 cm (c) 40 cm (d) infinity

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