An object is placed 20 cm from a convex lens of focal length 10 cm. Using the lens formula, what is the image distance $v$?

1. Lens formula: $\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$, with $f = +10$ cm and $u = -20$ cm. 2. Substitute: $\dfrac{1}{v} = \dfrac{1}{f} + \dfrac{1}{u} = \dfrac{1}{10} + \dfrac{1}{-20}$. 3. Compute: $\dfrac{1}{v} = \dfrac{2 - 1}{20} = \dfrac{1}{20}$. 4. So $v = +20$ cm, a real image on the far side. 5. The value $+10$ cm wrongly equals $f$ and ignores the object term, so it is a trap. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Lens Formula", p.29_