Lens formula
- CBSE · NCERT Class 10, Ch 9 Light - Reflection and Refraction
The lens formula, 1/v - 1/u = 1/f, links the object distance, the image distance and the focal length of a spherical lens.
All three distances are measured from the optical centre of the lens. The formula works for convex and concave lenses alike. Note the minus sign. The mirror formula adds 1/v and 1/u, but the lens formula subtracts. Keep the two apart.
A convex lens has a positive focal length. A concave lens has a negative one. The object distance u is negative, because the object sits on the left.
A small example
A convex lens has a focal length of 10 cm. An object is placed 15 cm from it. So f = +10 cm and u = -15 cm.
- 1/v = 1/f + 1/u = 1/(+10 cm) + 1/(-15 cm).
- 1/v = 3/(30 cm) - 2/(30 cm) = 1/(30 cm).
- v = +30 cm. The image is real and on the other side of the lens.
Where marks go
Question 35 of CBSE's 2026-27 Science sample paper tests this formula. In the version for visually impaired students, a convex lens of focal length 12 cm has an object 24 cm away. The 2026-27 Science marking scheme gives 2 marks for v = 24 cm. It gives 1 more mark for saying the image is real, on the other side of the lens.
So a correct number is not the whole answer. The sentence that reads the sign is worth a mark of its own. Read how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
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