Question

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

Answer 1

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