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NEET PHYSICSRAY OPTICS AND OPTICAL INSTRUMENTSMedium

Question

In the figure shown here, what is the equivalent focal length of the combination of lenses? (Assume that all layers are thin)

A

-50 cm

B

40 cm

C

-40 cm

D

-100 cm

Step-by-Step Solution

  1. Identify Components: The problem typically involves a combination of lenses (e.g., a convex lens and a liquid layer or multiple solid layers) placed in contact. The equivalent focal length (feqf_{eq}) is found by treating each layer as an individual thin lens.
  2. Lens Maker's Formula: Calculate the focal length (fif_i) of each individual lens using the formula: 1fi=(μi−1)(1R1−1R2)\frac{1}{f_i} = (\mu_i - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) where μi\mu_i is the refractive index of the layer and R1,R2R_1, R_2 are the radii of curvature of its surfaces (using proper sign convention).
  3. Combination Formula: For thin lenses in contact, the equivalent focal length is given by: 1feq=1f1+1f2+…\frac{1}{f_{eq}} = \frac{1}{f_1} + \frac{1}{f_2} + \dots
  4. Calculation: By substituting the specific values of μ\mu and RR from the missing figure into the Lens Maker's formula for each layer and summing their powers, one would arrive at the net focal length. Based on the provided probable answer, the result is −100 cm-100 \text{ cm}.

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from RAY OPTICS AND OPTICAL INSTRUMENTS. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSRAY OPTICS AND OPTICAL INSTRUMENTSfigureequivalentlengthcombinationlenses

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