A laboratory demonstration employs a single thin lens whose focal length is unknown
PHYSICS · Multi-part · CBSE · Physics · Light — Reflection and Refraction · Light — Reflection and Refraction
Problem
A laboratory demonstration employs a single thin lens whose focal length is unknown. An object slide is successively positioned at five different distances from the lens: much greater than 2f, slightly greater than 2f, exactly at 2f, between f and 2f, and inside f (where f is the unknown focal length). At each position, observers record only qualitative information about the resulting image (real or virtual, upright or inverted, enlarged or reduced).
Assume the lens may be either converging or diverging, but its type is not disclosed to the observers.
(1) Explain how the observers could determine the lens type after completing their set of qualitative observations at all five object positions.
(2) Once the lens type is identified, describe how the pattern of image orientation (upright or inverted) across the five object positions leads to an inference about whether the lens’s focal length is positive or negative.
(3) For the identified lens type, predict which of the five object positions yields an image that is the same size as the object. Provide a qualitative justification based on the behaviour of construction rays.