Power P1 = 2D;
Since Power is positive. Therefore, the lens is convex lens.
Power = \(\frac{1}{focal\,length(in\,meters)}\)
⇒ 2 = \(\frac1f\)
⇒ f = \(\frac12\) = 0.5m
1m = 100cm
⇒ 0.5m = 50cm.
Hence the focal length is 50cm
P2 = -4D.
Since the power is negative. Therefore, the lens is concave lens.
Power = \(\frac{1}{focal\,length(in\,meters)}\)
⇒ -4f = \(\frac1f\)
⇒ f = \(\frac1{-4}\) = -0.25m
1m = 100cm
⇒ 0.25m = 25cm.
Hence the focal length is -25cm