1. What is reflection?

When light traveling in one medium reaches the surface of another, part of it returns to the original medium — that is reflection. We see non-luminous objects because light reflected from their surfaces reaches our eyes.

2. Basic terms

Note: i and r are measured from the normal — never from the mirror face.

• Incident ray AO: the ray toward the reflecting surface.
• Reflected ray OB: the ray leaving the surface after reflection.
• Normal ON: through the point of incidence, perpendicular to the surface (usually dashed).
• Angle of incidence i: between the incident ray and the normal.
• Angle of reflection r: between the reflected ray and the normal.
• Point of incidence O: where the incident ray meets the surface.

3. Core lab: exploring the law of reflection

Purpose: find the rules of reflection — how the reflected ray, incident ray, and normal are arranged, and how r relates to i.

Kit: plane mirror, foldable stiff card, laser/source, protractor, pen, ruler. The foldable card (or screen) both shows the ray path and tests whether the three lines are coplanar.

3 (cont.). Procedure

Record a table: trial | i | r. After several trials, state the pattern.

1. Step 1 (positions): Place the mirror on a level table; stand the protractor card vertical on the mirror (line ON perpendicular to the glass). Send a beam along the card to point O, watch the reflected ray, and trace both rays on the card.
2. Step 2 (angles): Change the incident direction (vary i); record i and r several times and compare them.
3. Step 3 (coplanar): Fold the card’s reflected-ray half forward or back about ON. If the reflected ray vanishes from the card, the three lines were coplanar.

4. Conclusion — the law of reflection

• Coplanar: reflected ray, incident ray, and normal lie in one plane.
• Opposite sides: reflected and incident rays lie on opposite sides of the normal.
• Equal angles: angle of reflection equals angle of incidence.

• i = rSay “r equals i,” not the reverse — r is determined by i (cause → effect).

5. Reversibility of the light path

Send light back along the reflected path and it returns along the original incident path — reflection is reversible.

Everyday example: if A sees B’s eyes in a plane mirror, B can also see A’s eyes in the same mirror.

6. Specular vs diffuse reflection

• Specular: smooth surfaces (mirrors, still water, polished metal). Parallel in → parallel out. Examples: glare on a blackboard, “light pollution,” glitter on water.
• Diffuse: rough surfaces (paper, walls, cloth, screens). Parallel in → scattered out — so we see objects from many directions.
• Both obey the law of reflection. In diffuse reflection each ray still has i = r; only the local normals differ.
• Apps: specular → periscope, solar cooker, reflecting telescope; diffuse → rough cinema screen (visible from many seats), matte classroom board (less glare).

7. Ray construction

1. Given one ray and the mirror, draw the other: (1) erect the normal (dashed, right-angle mark); (2) on the other side of the normal set the angle equal and add an arrow.
2. Given both rays, find the mirror: (1) bisect the angle between them — that is the normal; (2) through O draw a line perpendicular to the normal — that is the mirror.

8. Pitfalls & exam tips

• i is from the normal. If the ray makes 30° with the mirror face, then i = 90° − 30° = 60°, and r = 60° too.
• If the angle between incident and reflected rays is 120°, then i = 60° (since 2i = 120°).
• Ray–mirror angle 30° means i = 60°. Move the incident ray 10° toward the normal → i = 50°, and the angle between the two rays drops from 120° to 100° (down 20°).
• The normal is always perpendicular to the mirror and bisects the angle between the two rays.
• Coplanarity test: after folding the screen half, if the reflected ray disappears, the three lines were coplanar.

9. Applications in life

• Daily life: seeing non-luminous objects, looking in mirrors, car rear-view mirrors.
• Traffic: bicycle reflectors, retroreflective road markings.
• Military: periscopes (plane-mirror reflection); stealth shapes that scatter radar (diffuse-like returns).
• Energy: solar water heaters, tower CSP plants (mirrors concentrate sunlight).
• Tech: laser ranging; total internal reflection in fiber optics is a special case of reflection (met later in the course).