The dispersive powers of the materials of two lenses forming an achromatic combination are in the ratio of 4: 3 . Effective focal length of the two lenses is +60 ~cm then the focal lengths of the lenses should be

Step-by-step solution

aligned P=P₁+P₂ & = q f₁ + q f₂ 1 60 & = q f₁ + q f₂ 1 60 & = f₁+f₂ f₁ f₂ aligned According to question, ₁ ₂ = f₁ f₂ f₁ f₂ = -4 3 or f₁=- 4 3 f₂ From Eq. (i) 1 60 = 4 3 f₂+f₂ (f₂ )^2 4 3 - 1 3 - 4 3 f₂ aligned f₂ & =15 ~cm f₁ & =- 4 3 15 & =-20 ~cm aligned