Ray diagram
Also called Ray tracing
A ray diagram is a sketch of the path light takes before and after it meets a mirror or lens, drawn with straight rays, and used to find where an image forms and what it looks like.
Essential knowledge 13.1.A.2 puts it in one line: ray diagrams depict the path of light before and after an interaction with matter. What makes the sketch legitimate is EK 13.1.A.1, which defines a light ray as a straight line perpendicular to the wavefront and pointing the way the wave travels.
What the diagram is for. EK 13.2.A.9 and EK 13.4.A.7 give the same job to mirrors and lenses: ray diagrams can be used to determine the location, type, size, and orientation of images. Those four readings map onto the four answers an image question wants, since an image can be upright or inverted, virtual or real, and reduced, enlarged, or the same size as the object (13.2.A.9.ii, 13.4.A.7.ii).
The principal rays. The CED says three are typically used, and lists them for each optic. For a lens (13.4.A.7.i): the ray parallel to the principal axis, the ray that passes through the centre of the lens where the principal axis intersects it, and the ray that passes through the focal point. For a mirror (13.2.A.9.i) the middle one instead reflects at the centre of the mirror where the principal axis meets it. Any two of them fix the image point; the third is a check on your drawing.
Where rays run out. EK 13.1.A.1.i permits them only where the wave nature of light can be neglected, and EK 13.1.A.1.ii says outright that rays are not sufficient to understand the spreading of light, because in interference and diffraction the wave nature matters. Draw rays in Unit 13, draw wavefronts in Unit 14.