Light — Reflection

• Reflection of light — light bounces off a surface; angle of incidence = angle of reflection.
• Laws of reflection — two laws and how to draw the normal, incident ray, reflected ray.
• Difference between regular (specular) and irregular (diffuse) reflection.
• Images in a plane mirror — properties: virtual, erect, laterally inverted, same size, same distance behind mirror.
• Periscope working principle — two plane mirrors at 45°.

Level 1 — Laws of reflection

1. Incident ray — ray of light hitting a surface.
2. Normal — perpendicular line at the point of incidence (drawn with dashed line).
3. Angle of incidence (i) — angle between incident ray and normal.
4. Angle of reflection (r) — angle between reflected ray and normal.
5. First law — incident ray, normal, and reflected ray all lie in the same plane.
6. Second law — angle of incidence = angle of reflection (∠i = ∠r).

Level 2 — Types of reflection

Diffuse reflection scatters light in many directions — this is why we can see objects that don't glow.

Level 3 — Plane mirror image

• Image distance = object distance (behind mirror).
• Image is virtual (cannot be caught on screen).
• Image is erect (right-side up).
• Image is laterally inverted — left becomes right (ambulance written reversed).
• Image size = object size.

Periscope: two mirrors at 45° each deflect light 90° → total 180° change; used in submarines.

Angle of incidence is 35°. Find angle of reflection and the angle between reflected ray and mirror.

Step 1 — ∠i = 35° (given).
Step 2 — By second law: ∠r = ∠i = 35°.
Step 3 — Angle between reflected ray and mirror surface
         = 90° − ∠r = 90° − 35° = 55°.
Step 4 — Total angle between incident and reflected rays
         = ∠i + ∠r = 35° + 35° = 70°.
Conclusion: angle of reflection = 35°; ray makes 55° with mirror surface.

• State the two laws of reflection.
• If ∠i = 40°, what is ∠r?
• Why does an ambulance have reversed writing on its front?
• What is diffuse reflection? Give two examples.
• Draw a ray diagram for a plane mirror (hint: mark normal, ∠i, ∠r).

Open the Practice tab for graded questions on Light — Reflection.