Physics - Error Question with Solution | TestHub

The correct answer is C because interchanging the object and image needle is NOT a necessary precaution. The other options are important for accurate focal length determination: keeping the image needle at a reasonable distance (around 25 cm) for clear viewing, aligning the needle tip with the lens's optical center to minimize parallax error, and accurately removing parallax by ensuring the tip of the image needle coincides with the image formed by the lens.