Practice portal › Experimental Skills › Focal Length of Mirrors and Lenses
Asked in Re-NEET 2026 · Focal length on the optical bench
On an optical bench the positions of the lens, object needle and image needle are read from the scale, but the scale reading between two uprights need not equal the true distance between the needle tip and the lens.
The difference, found once with an index rod of known length, is the index (bench) correction, applied to every u and v.
So the optical-bench measurement of focal length is the one that needs it.
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