Two lenses of power +3.5 D and -1.5 D are placed in contact. What is the power and focal length of the combination ? If an object be placed at a distance of 0.8 m from the lens combination, where is the image formed ?

Two lenses of power +3.5 D and -1.5 D are placed in contact. What is t

1.	Two lenses of power +3.5 D and -1.5 D are placed in contact. What is the power and focal length of the combination ? If an object be placed at a distance of 0.8 m from the lens combination, where is the image formed ?
Answer» Solution :It given that `P_(1) = + 3.5 D, P_(2) = - 1.5 D` `therefore` of the combination `P = P_(1) + P_(2) = (+3.5 - 1.5) D = + 2.0`D `therefore` FOCAL length of the combination `f = (1)/(P) = (1)/(+2.0 D) = + 0.5`m As distance of OBJECT from the lens combination u = - 0.8 m, hence using lens formula `(1)/(v) - (1)/(u) = (1)/(f)`, we have `(1)/(v) = (1)/(u) + (1)/(f) = (1)/((-0.8)) + (1)/((+0.5)) = (-0.5 + 0.8)/(0.8 xx 0.5) = (0.3)/(0.8 xx 0.5) rArr v = (0.8 xx 0.5)/(0.3) = + 1.33` m