Rectilinear propagation of light means that light travels in straight lines.

An object is able to be seen if it emits light or reflects light.

A ray is a line representing the direction in which light is travelling.

A beam of light is a stream of light energy and is represented by a number of rays.

Optical density is an indication of the speed at which light travels through a medium. The greater the optical density, the smaller the speed of light. Light travels at 3 x 10⁸ m.s^-1 in a vacuum.

When light enters glass its speed diminishes. If the light ray enters the glass obliquely its speed changes and so does its direction. We say that the light is refracted. When light enters a less optically dense medium it is bent away from the normal. When it enters a medium that is optically denser it is bent towards the normal. When light falls perpendicularly onto the interface between the two media its speed changes, but it does not change direction.