Law of reflection: i = r from the normal
The law of reflection: the angle of incidence equals the angle of reflection , and the incident ray, reflected ray and normal lie in one plane. Both angles are measured from the normal — the line perpendicular to the surface at the point of incidence — never from the surface. It holds for every ray on any surface, smooth or rough. Mini example: a ray meeting a mirror at 30° to the surface has , so .
Plane mirror image + specular vs diffuse
A plane mirror image has five properties (mnemonic VUSLD): Virtual (cannot be projected on a screen), Upright, Same size, Laterally inverted (left and right swapped), Distance behind the mirror = object distance in front. Specular reflection (smooth surface) keeps parallel rays parallel → a sharp image; diffuse reflection (rough surface) scatters rays in all directions → no image, though the surface stays visible. holds at each point.
Drawn from real examiner reports.
Angle measured from surface, not normal
The top error: measuring from the surface instead of the normal. A ray 40° from the normal is 50° from the surface — a 10° error, outside diagram tolerance. Mark schemes always say "measured from the normal". Fix: draw the dashed normal first, then measure your angle to it. The same slip cost refraction-angle marks in June 2023 Paper 2P Q7.
June 2023 Paper 2P Q7(a)(ii)–(iv) — examiner report noted candidates measuring angles from the surface rather than from the normal on refraction diagrams; this systematic error applies identically to reflection angle questions.
Calling the mirror image "real"
A plane mirror image is virtual, not real. Definition: (1) it cannot be projected onto a screen, and (2) it only appears behind the mirror, where no light converges. A real image forms where rays meet and CAN be projected (e.g. a lens beyond its focal point). Writing "real", or "behind the mirror" without the "cannot be projected" element, loses the mark.
"Inverted" is not "laterally inverted"
The image is upright (same way up), NOT inverted. What flips is left ↔ right — this is lateral inversion. Letters look "backwards" because of lateral inversion, not a vertical flip. The required pair is upright AND laterally inverted. Reserve plain "inverted" (upside-down) for the real image formed by a converging lens.
June 2023 Paper 2P and November 2023 Paper 2P — examiner reports note that candidates describing mirror images lose marks for writing "inverted" without the qualifier "laterally"; the mark scheme distinction between upright and laterally inverted is precise.
Thinking rough surfaces break i = r
Do not think diffuse reflection breaks the law of reflection — still holds at every point. On a rough surface each tiny facet has a differently angled normal, so parallel incident rays reflect at many angles and scatter. That is why no clear image forms, even though each individual ray obeys .
Normal drawn solid or along the surface
The normal must be a dashed line drawn perpendicular to the mirror at the point of incidence. Drawing it solid, or parallel to the surface, loses the normal mark and throws off every angle. Draw the normal first, before any ray, then measure and to it.
Angle between rays taken as 180°
Asked for the angle between the incident and reflected rays, some add to 180° (treating them as a straight line) or quote alone. The two rays are symmetric about the normal, so the angle between them is . Example: gives , not 180°.
(general exam technique)
Ray diagram: normal, incident, reflected
Full-mark ray diagram, in order: (1) draw the dashed normal perpendicular at the point of incidence; (2) draw the incident ray, arrow towards the mirror; (3) draw the reflected ray at the equal angle on the far side of the normal, arrow away. Label and .
List all five image properties (VUSLD)
"Describe the image" expects every property — each is a separate mark. Work through VUSLD: virtual, upright, same size, laterally inverted, distance behind equals distance in front. Never stop at just one or two.
Label and title every diagram
Label everything. The November 2023 Paper 2P report told candidates to label and title diagrams: a correctly drawn but unlabelled ray still loses the identification mark. Label the incident ray, reflected ray, normal and the angles and .
Convert surface angle before applying i = r
If the question gives the angle to the surface, convert first: , then apply . Skipping this conversion is the biggest single source of lost angle marks in reflection questions.
The law of reflection (no formula sheet entry — must be recalled):
where = angle of incidence (degrees, °) and = angle of reflection (degrees, °).
Both angles are measured from the normal — an imaginary dashed line drawn perpendicular to the reflecting surface at the point where the ray meets the surface.
There is no rearrangement to learn: if you know you immediately know .
Image distance relationship for a plane mirror:
The image is as far behind the mirror as the object is in front. No formula-sheet entry; recall directly.
State the law of reflection.
A ray of light strikes a plane mirror. The ray makes an angle of 25° with the mirror surface. Calculate the angle of incidence.
