Question

Find the position of final image from first lens.If the focal length of each lens is 10 cm.

• A. 40 cm
• B. 50 cm
• C. 45 cm
• D. 55 cm

Answer 1

The Correct Option is B

Refraction from lens A, From lens formula, $\frac{1}{V_{A}}-\frac{1}{u_{A}}=\frac{1}{f_{A}}$ $\frac{1}{V_{A}}=\frac{1}{f_{A}}+\frac{1}{u_{A}}$ $\frac{1}{V_{A}}=\frac{1}{10}-\frac{1}{40}$ $V_{A}=\frac{40}{3}$ cm Refraction from lens B, v is object for lens B, $u_{B}=30-\frac{40}{3}=\frac{50}{3}$ cm So, $\frac{1}{v_{B}}-\frac{1}{u_{B}}=\frac{1}{f_{B}}$ $\frac{1}{v_{B}}+\frac{3}{50}=\frac{1}{10}$ $\frac{1}{v_{B}}=\frac{1}{10}-\frac{3}{50}$ $\frac{1}{v_{B}}=\frac{2}{50}$ $v_{B}=25$ cm Refraction from lens C, v is object for lens C, u = 30 - 25 = 5 cm $\frac{1}{v_{C}}=\frac{1}{f_{C}}+\frac{1}{u_{C}}\Rightarrow \frac{1}{v_{C}}=\frac{1}{10}-\frac{1}{5}\Rightarrow \frac{1}{v_{C}}=\frac{-1}{10}$ $v_{c}=-10$ cm Final image distance from lens C is 10 cm towards lens B. So the final image distance from lens A is, = 20 + 30 = 50 cm