Drawing Ray Diagrams

For ages 12–13, this topic is the drawing-accuracy craft of constructing optical ray diagrams: plane-mirror reflection (incident ray, normal, reflected ray with measured angle of reflection equal to angle of incidence, and a dashed virtual image behind the mirror located where two reflected rays' back-extensions meet), refraction through a rectangular glass block (an oblique ray bends toward the normal on entering the glass and away from it on leaving; a ray along the normal does not bend), and a convex-lens principal-ray construction that locates an image and names it real or virtual. The central disciplines are: draw the dashed normal first (perpendicular at the point of incidence) and measure every angle from the normal, never from the surface; use conventions deliberately (ruled straight rays with arrowheads, solid for real light paths, dashed for the normal and virtual/construction extensions); and use two rays to locate an image point. The authentic task is hand-drawing with a physical protractor and ruler on paper with teacher feedback; a guided on-screen construction editor is a strong stand-in for the concepts but does not fully replicate the paper-and-protractor motor skill.

• Measuring angles from the mirror/surface instead of from the NORMAL — the signature drawing error; produces a reflected ray at the wrong angle.
• Omitting the normal altogether, or drawing it not perpendicular / not at the point of incidence.
• Drawing rays freehand or curved instead of ruled straight lines, and leaving off arrowheads so direction of travel is lost.
• Using a SOLID line for the virtual image's back-extension (it should be DASHED) — conflating a real light path with a construction line.
• Refraction drawn the wrong way — bending away from the normal when entering glass, or drawing a curved path rather than a sharp change of direction exactly at the boundary.
• Eyeballing 'equal angles' instead of measuring, so the reflected ray is visibly unequal; sloppy scale/ruler work.
• Not knowing which or how many rays to draw — thinking one ray suffices, or picking rays that don't locate the image.
• Carrying over the real/virtual confusion — drawing the plane-mirror image as if real, or expecting a virtual image to sit on a screen.

• The dashed NORMAL with a protractor centred at the point of incidence — the reference from which i and r are measured.
• Plane-mirror construction: incident ray, normal, reflected ray (measured i = r), and dashed back-extensions meeting at the virtual image behind the mirror.
• Single-boundary refraction diagram: the normal at the boundary and the ray bending TOWARD the normal on entering glass (a sharp change of direction, not a curve).
• Convex-lens principal-ray construction locating the image and labelling it real or virtual.
• The solid / dashed / arrowhead convention itself — the notation that distinguishes real light path from construction/virtual line and shows direction of travel.
• The physical toolkit: ray box + plane mirror / glass block + protractor + ruler — what the accurate diagram is drawn to match.

• NORMAL FIRST, ALWAYS: draw the dashed normal perpendicular AT the point of incidence before any reflected/refracted ray, and measure BOTH angles from the normal — never from the surface. This single habit prevents the commonest construction error.
• Protractor discipline: place the protractor centre exactly at the point of incidence, read to whole degrees, and CHECK angle of reflection = angle of incidence by measuring, not eyeballing 'looks about equal'.
• Teach the conventions as non-negotiable notation: ruled straight lines (never freehand curves), ARROWHEADS showing the direction light travels, SOLID lines for real light paths and DASHED lines for the normal and for virtual/construction extensions.
• Use the two-ray principle explicitly: only two rays are needed to locate an image point — teach WHICH two (any two incident rays for a mirror; the principal rays for a lens) so pupils aren't guessing.
• Predict–observe–reconcile against a real ray box: have pupils DRAW the predicted diagram first, then shine a ray box + place a mirror/glass block and overlay the real rays, reconciling any discrepancy — grounding the drawing in the phenomenon.
• One scenario at a time as a repeatable template: master mirror-reflection, then single air→glass refraction (bend toward the normal), then the convex-lens principal rays — each as its own drilled routine before combining.
• Insist on full labelling as part of the construction: incident ray, normal, angle i, angle r, reflected/refracted ray, and image — an unlabelled diagram is an incomplete answer.
• Show a correct worked exemplar beside the pupil's attempt so they can self-check against the conventions (normal drawn? measured from normal? dashed for virtual? arrowheads?).