Light and Optics
Light and Optics tests reflection by plane and spherical mirrors, refraction through glass and lenses, image formation, and basic ideas of dispersion and the human eye. Mirror and lens formula numericals are common.
Core ideas
- Laws of reflection: angle of incidence equals angle of reflection; incident ray, reflected ray and normal lie in one plane.
- Concave mirrors can form real or virtual images; convex mirrors always form virtual, erect, diminished images and have wider fields of view.
- Mirror formula: 1/f = 1/v + 1/u; magnification m = -v/u. Use the Cartesian sign convention.
- Refraction: light bends because its speed changes between media. Snell's law: n = sin i / sin r.
- Lens formula: 1/f = 1/v - 1/u; power P = 1/f (in metres), unit dioptre.
- Convex lens converges; concave lens diverges. A convex lens can form real or virtual images.
Worked example
An object is 20 cm in front of a concave mirror of focal length 10 cm. Using u = -20 cm and f = -10 cm: 1/v = 1/f - 1/u = -1/10 + 1/20 = -1/20, so v = -20 cm. The image is 20 cm in front of the mirror, real and inverted. Magnification m = -v/u = -(-20)/(-20) = -1, so it is the same size.
Traps that cost marks
Sign errors are the biggest cause of lost marks. Distances measured against the incident light are negative in the standard convention. Do not call a virtual image unreal because it cannot be caught on a screen; it can still be seen. Magnification negative means inverted, not diminished.