The lens formula: where the image forms

For a convex lens 1/v − 1/u = 1/f: an object beyond the focus F gives a real, upside-down image on the other side; an object inside F gives a virtual, upright, bigger image on the same side

Topic: Light: refraction and lenses

A convex lens, like a magnifying glass, is thicker in the middle. Rays parallel to its axis bend and meet at the focus F, a focal length f from the lens. An object at a distance u in front of the lens forms an image at a distance v, given by the lens formula 1/v − 1/u = 1/f. In the NCERT sign convention distances are measured from the centre of the lens, so u in front of the lens is negative; with that, v = fu ÷ (u − f) when u is the plain distance. A positive v means a real image on the other side, which can be caught on a screen and is upside down. A negative v means a virtual image on the same side as the object, upright and larger, seen by looking through the lens. The magnification m = −v ÷ u tells how many times bigger the image is; a minus sign means upside down.

Key words explained