Lens Formula
The lens formula relates object distance, image distance, and focal length for a thin lens.
Practice This ConceptMain explanation
Teacher explanation
For class 10, use 1/f = 1/v - 1/u with Cartesian sign convention. It works for both convex and concave lenses if signs are applied correctly. The formula helps predict whether the image will be real or virtual and how far it will form.
Example
For a convex lens with f = 15 cm and u = -30 cm, the image distance is 30 cm.
Simple analogy
Lens formula is mirror formula with its own sign pattern.
Common confusion
Students often use 1/f = 1/u + 1/v or forget that object distance is negative in front of the lens.
Exam tip
A real object is usually placed on the left side, so u is negative in the Cartesian sign system.
Answer writing and exam use
1-mark use
Write the exact meaning of lens formula in one clean line.
2-mark use
Define lens formula and add one example or condition.
3-mark use
Explain lens formula, show the method or example, and mention the common mistake.
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