Magnification
Definition
Magnification tells you how much bigger (or smaller) an image is than the object — a microscope's 40× means the image is 40 times the object's size.
Symbol: ·
Technical definition
Lateral magnification m = −dᵢ/dₒ = hᵢ/hₒ. The negative sign means the image is inverted (real images); positive means upright (virtual images). |m| > 1 enlarges, |m| < 1 reduces.
Example
A projector with the screen image 60 cm from a lens and the slide 12 cm away gives m = −5 — an inverted image 5 times larger than the slide.
Common confusion
Magnification of −1 is not 'negative size' — the minus only records that the image is upside down. And angular magnification (how big something looks to your eye) is a different quantity from lateral magnification.