Light and Optics — Scientific Principles

Scientific Principles

Keep the reference line explicit

The angle of incidence and angle of reflection are measured from the normal, not the reflecting surface. A ray at 30° to a mirror surface is at 60° to its normal. Reflection preserves equality of those normal-based angles. A plane mirror's virtual image is upright and equal in size to the ordinary object.

Link speed, wavelength and index

Refractive index compares vacuum light speed with phase speed in a medium at a specified wavelength. At a stationary boundary, frequency stays unchanged while speed and wavelength can change. An illustrative 600 nm vacuum wavelength becomes 400 nm in a medium of index 1.5. At normal incidence the ray need not bend even though its speed changes.

State the conditions for guiding

Total internal reflection requires travel from higher to lower index and incidence above the critical angle. Optical fibres use an appropriate core–cladding index relationship, but real transmission still has losses. Guiding is one part of a complete communication system.

Calculate before naming the image

For a thin lens, use one sign convention throughout. With f = +20 cm and u = −30 cm, the Cartesian relation gives v = +60 cm and magnification −2. The positive image distance and negative magnification answer different questions: location and orientation. Lens power uses focal length in metres, so +0.20 m corresponds to +5 dioptres.