Solution

Object distance, u =∞ (Infinity) Image distance, v = -80 cm (Far point, in front of lens) Focal length, f=? Using lens formula: 1/v -1/u = 1/f We get 1/-80 - 1/∞ = 1/f or -1/80 -1/∞ =1/f (1/∞ = 0) f=-80 Thus, the focal length of the required concave lens is 80 cm. To find power, we use Power,P = 1/f =1/-0.8 =-10/8 P = -1.25D Therefore, the power of concave lens required is, -1.25 dioptres.