Given, two lenses. (i) Here, Power, P = + 2D ∴ Focal length, f = 1P = 1+2m = + 50 cm. 
Since, the focal length is positive, the lens is convex.      (ii) Here, Power, P =  - 4D ∴ Focal length, f = 1P = 1-4m = -25 cm.  
Negative sign of the focal length implies that the lens is concave.