Lenses and optical instruments · Grade 10
Lenses refract light to form images. A converging (convex) lens brings parallel rays together at its principal focus; a diverging (concave) lens spreads them out. Lenses are in cameras, glasses, microscopes, projectors and your own eyes. The lens equation and magnification tell you exactly where an image will form and how big it will be.
Key facts
- Converging lens: thicker in the middle; parallel rays meet at the principal focus, a distance f from the lens.
- Diverging lens: thinner in the middle; it makes rays spread as if from a focus.
- Magnification = image height ÷ object height = v ÷ u.
- Lens equation (real is positive): 1/f = 1/u + 1/v; a virtual image has negative v.
- Power P = 1 ÷ f (dioptres, f in metres); converging +, diverging −. Powers of thin lenses in contact add.
- Object beyond 2f: real, inverted, smaller (camera). Between f and 2f: real, inverted, bigger (projector). Inside f: virtual, upright, bigger (magnifying glass).