1.find the focal length and nature of plane which should be contact with lens of focal length 10cm. the power of lens is 5 dioptre.
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Two key facts:
The power of a lens is the reciprocal of the focal length in meters.
Powers are additive.
The lens with a focal length of +20 cm has a power of 1/0.2 = +5.
The lens with a focal length of -15 cm has a power of 1/(-0.15 ) = -6.67
The two lenses together have a power of 5 - 6.67 = -1.67.
The focal length of the combination is 1/(-1.67) = -0.6 or -60 cm.
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