Question

Two thin lenses have combined power of 10 D. When separated by 20 cm their equivalent power is 6.25 D. Their individual power in D are –

• A. 3.5 and 6.5
• B. 5 and 5
• C. 7.5 and 2.5
• D. 9.0 and 1

Answer 1

Answer: (C)

7.5 and 2.5

Answer 2

P₁ + P₂ = 10 D

P₁ + P₂ – $\frac { 1 } { 5 }$P₁ P₂ = 6.25 D

$\frac { 1 } { 5 }$P₁ P₂ = 3.75

P₁ P₂ = 18.75

P₁ – P₂ = = $\sqrt { 100 - 75 }$ = 5

P₁ + P₂ = 10 Ž P₁ = $\frac { 10 + 5 } { 2 }$= 7.5D & P₂ = 2.5D