Question

Two lenses of power -3.5 D and + 1 D are placed in contact. Find the total power of the combination of lens. Calculate the focal length of this combination.

Answer 1

Power P₁ = -3.5D; P₂ = +1D 

Total power = P₁ + P₂ = -3.5D + 1D 

⇒ Total power (P) = -2.5D. 

Now; 

P (in diopter) = \(\frac{1}{f(in\,meters)}\)

⇒ -2.5 = \(\frac{1}{f(in\,meters)}\)

⇒ f (in meters) = \(\frac{-1}{2.5}\,=\,\frac{-1}{\frac{5}{2}}\)

= f (in meters) = - \(\frac25\)= -0.4m

Now 1m = 100cm 

⇒ -0.4m = -40cm 

Hence the focal length of lens is -40cm.