Reflection of waves — Required practical 9
When a wave hits a boundary, some or all of it is reflected back into the original medium.
Law of reflection
The angle of incidence equals the angle of reflection.
Both angles are measured from the normal — a line drawn perpendicular to the surface at the point of contact.
- Incident ray — incoming wave.
- Reflected ray — outgoing wave.
- Normal — perpendicular to surface.
- The incident ray, normal, and reflected ray all lie in the same plane.
Specular vs diffuse reflection
- Specular (mirror-like) — smooth surface; parallel incoming rays reflect parallel outgoing.
- Diffuse — rough surface; parallel incoming rays scatter in many directions. (Why we see things from any angle, not just at a perfect mirror angle.)
Required practical 9 — measuring reflection
Apparatus: ray box, slit, plane mirror, protractor, white paper.
Method:
- Place mirror flat on paper; mark its position.
- Direct ray from box at the mirror; mark incident ray with two crosses.
- Mark the reflected ray (two crosses).
- Remove mirror; draw incident, normal, reflected lines.
- Measure angle of incidence and angle of reflection from the normal.
- Repeat for several angles.
Result: angle of incidence = angle of reflection (within experimental error).
Reflecting different waves
- Light — mirrors, white surfaces, etc.
- Sound — echoes from walls; ultrasound for medical/industrial scanning.
- Water waves — bounce off the side of a tank.
- Seismic waves — reflect off internal Earth boundaries (used to map crust).
⚠Common mistakes
- Measuring angle from the surface instead of the normal.
- Forgetting that smooth and rough surfaces both obey the law — for rough, each tiny "facet" reflects according to its own normal.
- Drawing the normal in the wrong place.
- Confusing reflection with refraction (refraction is bending into a new medium).
AI-generated · claude-opus-4-7 · v3-deep-physics