Speed of light : 3 x 108 m/s in a vacuum or in air. Speed of any EM spectrum wave. Can slow down to smaller value if wave enters more optically dense medium.
Electromagnetic radiation : Transverse waves of pure energy, with no mass or charge and that travel at the speed of light. They do not require a medium to travel through.
Dispersion : Property of waves spreading apart from each other. Dispersion happens inside a prism as white light refracts into different colours.
Centre of curvature : Twice the focal length. C = 2f = radius of the curve of the surface of a convex/concave mirror or from 1 side of convex/concave lens.
Converging : Type of mirror or lens that reflects/refracts parallel waves to cross (converge) at a focal point.
Diverging : Type of mirror/lens that reflects/refracts parallel waves to spread further and further apart (as if they each started from a focal point).
Focal length : Symbol "f" is used in equations. The distance from the focal point to the core/centre of the curved mirror/lens.
Focal point : Location on the principal axis where parallel light rays converge after getting refracted by curved lens or reflected by curved mirror.
Magnification : Numerical value of how much an image's height/size is different than object's.
Principal axis : Imaginary line through the "middle" or "core" of a lens/mirror wave. Usually horizontally across a diagram.
Reflection : When a wave changes direction by bouncing off a surface. Angle of reflection = angle of incidence (both measured to the normal, not the surface).
Refraction : The changing of a wave's direction because of entering a new medium causing a change of speed and wavelength while keeping same frequency.
Total internal reflection : When light wave does not exit a medium after hitting boundary. Ray must be inside the more optically dense medium for TIR to happen.
Critical angle : When wave strikes boundary of two media and refracts at 90° along the surface. Ray must be inside the more optically dense medium.
Real image : Type of nature of image where light rays reach the image. Can be projected onto a screen as in a movie theatre or the retina of an eye
Virtual image : Type of nature of image where light rays do not reach the location of the image. In ray diagrams dotted lines are used to locate image.
Upright : Type of nature of image: compares orientation of image to object. If both are the same, then image is “upright" even if both are up-side-down.
Inverted : Type of nature of image: compares orientation of image to object where each is opposite. Usually reserved for "vertical inversion".
Diminished : Type of nature of image: compares height of image to object. If image is shorter/smaller than object.
Enlarged : Type of nature of image: compares height of image to object. If image is taller/larger than object.
Apparent depth : For light rays leaving water and entering observer's eye: virtual image of object will be above object making depth of water look less deep.
Laterally inverted : When an image has been "flipped left-to-right" compared to object. Commonly seen in writing on front of fire trucks or ambulances.