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Twot thin lenses when placed in contact,...

Twot thin lenses when placed in contact, then the power of combination is +10D. If they are kept 0.25m apart, then the power reduceds to +6D. The focal lengths of the lenses (in m) will be

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When the lenses are in contact
`P=P_1+P_2 or P_1+P_2=10`………(1)
When lenses have d separation `P_1+P_2=dP_1 P_2=P`
From (1) and (2) we get `P_1=8D, P_2=2D`
`therefore f_1=1/8=0.125 m , f_2=1/2 =0.5m`
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