Lenses of focal length 20cm and minus 25 cm are placed in contact find the power of combination
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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 -5 cm has a power of 1/(-0.25 ) = -4
The two lenses together have a power of 5 - 4 = 1
The focal length of the combination is 1/(1) = 1 or 100 cm.
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