Power P_{1} = 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

P_{2 }= -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