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    				| 1. | 
                                    Two thin lences of power +6D and-2D are in contact. What is the focal of the combination | 
                            
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Answer»  Power of combination= 6 D+(-2 D ) = 6 D -2 D = 4 D power = 1/focal length(m) 4 = 1/ff = 1/4 f = 0.25 metres f of combination = 0.25 * 100 = 25 cm  | 
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