Question

Power of convex lens is 5D and power of concave lens is 2D. What will be the power of the combination of these lens when placed in contact

Answer 1

Answer- The above question is from the chapter 'Light- Reflection and Refraction'.

Concept used: Power of combination of lenses- It is the algebraic sum of all powers of lenses kept in contact of each other.

P = P₁ + P₂ + P₃ + _ _ _ + Pₙ

where P = Total power

n = Number of lenses kept

Convex lens has a negative power.

Concave lens has a positive power.

Given question: Power of convex lens is 5 D and power of concave lens is 2 D. What will be the power of the combination of these lens when placed in contact?

Answer: P₁ = -5 D (being convex)

P₂ = +2 D

By Power of Combination of lenses, we get,

P = P₁ + P₂

P = - 5 + 2

P = - 3 D

∴ Power of combination of these lenses when kept in contact is - 3 D.

Answer 2

• Power of a given convex lens = 5 D
• Power of a given concave lens = 2 D

→ Combination of these lenses when placed together = ?

As we know,

◘ Convex lens has a negative power.

◘ Concave lens has a positive power.

So, we finally get :-

• Power of the convex lens = -5 D
• Power of the concave lens = 2 D

$\rule{150}2$

We know Power of combination of lenses :-

→ Power of Combination(P) = Power of the convex lens(P') + Power of the concave lens(P'')

⇒P = (-5) + 2

⇒P = -3 D

Therefore, the power of lenses when placed together is -3 D.