Sign convention for lens - Concave and Convex Lens - eSaral

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Sign Convention

Rules for image formation

In order to locate the image formed by a lens graphically following rules are adopted -

1. A ray passing through the optical center proceeds undeviated through the lens. (by definition of optical center).
   
2. A ray passing through the first focus or directed towards it, after refraction from the lens becomes parallel to the principal axis. (by definition of $F_{1}$)
   
3. A ray passing parallel to the principal axis after refraction through the lens passes or appears to pass through $\mathrm{F}_{2}$ (by definition of $\mathrm{F}_{2}$).
   
4. Only two rays from the same point of an object are needed for image formation and the point where the rays after refraction through the lens intersect or appear to intersect is the image of the object. If they actually intersect each other the image is real and if they appear to intersect the image is said to be virtual.
   

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