# Two lenses have power of (a) + 2D (b) – 4D. What is the nature and focal length of each lens?

8.9k views
in Physics
closed

Two lenses have power of (a) + 2D (b) – 4D. What is the nature and focal length of each lens?

+1 vote
by (54.2k points)
selected by

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