Lesson 2 of 11 · 11 min
The mirror equation and magnification
NCERT § "The Mirror Equation"
Kabir slides the candle towards his shaving mirror. On a card the flame appears upside down, grows, then vanishes, and suddenly he sees a large upright flame inside the mirror.
The lesson in notes
In short
Object distance u, image distance v and focal length f are related by 1/v + 1/u = 1/f, with all values taken with their signs.
Linear magnification of a mirror is m = h′/h = −v/u.
A negative m means an inverted image, which for a single mirror is real; a positive m means an erect image, which is virtual.
A concave mirror forms a real inverted image when the object is beyond the focus, and a virtual, erect, enlarged image when the object lies between the focus and the pole.
A convex mirror always gives a virtual, erect, diminished image located between the pole and the focus, for any real object.
For a concave mirror, an object placed at its centre of curvature gives a real inverted image of the same size, also at the centre of curvature.
Substitute signed values only once, into the formula; do not also insert signs by hand into the equation.