Question

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

Answer 1

a) +2 D

Given lens has a power of +2 Dioptre. It is known that the lens having a power of positive magnitude is convex lens. Hence, the lens used here is convex lens.

Focal length (in metres) is equal to the inverse of power.

Hence, f (in metres) = 1/P

→ f = 1/2 = 0.5 metres

→ f = 50 cm.

Focal length here is 50 cm.

$\rule{50}1$

b) - 4D

Here, since the lens has a negative power, the lens is concave lens.

Again, f (metres) = 1/P

→ f = 1/(-4) = -0.25 metres

→ f = -25 cm.

$\rule{200}1$

★ Convex lens is used for correction of hypermetropia (far sightedness)

★ Concave lens is used for correction of myopia (near sightedness)