Define and use the terms normal, angle of incidence and angle of reflection
3.2.1 Reflection of Light
Learning Objective
Define and use the terms normal, angle of incidence and angle of reflection; state and apply the law of reflection; construct accurate ray diagrams; describe the image formed by a plane mirror; and solve simple quantitative problems – exactly as required by Cambridge IGCSE Physics (0625) 3.2.1.
Key Formula
Law of Reflection: i = r (θ‑notation)
where i = angle of incidence, r = angle of reflection.
Key Definitions
- Normal: An imaginary line drawn perpendicular (90°) to the reflecting surface at the point where the incident ray meets the surface.
- Angle of Incidence (i): The angle between the incident ray and the normal.
- Angle of Reflection (r): The angle between the reflected ray and the normal.
Law of Reflection
- The incident ray, the reflected ray and the normal all lie in the same plane.
- Angle of incidence equals angle of reflection → i = r.
Exam‑style AO2 Question
Explain why the incident ray, the reflected ray and the normal must lie in the same plane.
Answer outline: Light travels in a straight line; the normal is defined at the point of incidence. Both the incident and reflected rays are measured from this normal, so all three share the same point and therefore define a single geometric plane. If they were not coplanar, the angles could not be measured consistently.
Ray‑Diagram Construction (Plane Mirror)
- Draw a straight horizontal line to represent the mirror surface.
- Mark the point of incidence (where the incident ray meets the mirror).
- At that point draw the normal – a short line perpendicular to the mirror.
- From the point of incidence draw the incident ray making the required angle i with the normal.
- From the same point draw the reflected ray on the opposite side of the normal such that r = i.
Practical Tip – Measuring i and r
- Place a small plane mirror on a flat board.
- Use a protractor and a narrow laser or ray box to produce the incident ray.
- Mark the point of incidence, draw the normal, and measure the angle between the incident ray and the normal (i).
- Measure the angle between the reflected ray and the normal (r).
- Repeat several times, calculate the average of |i − r| and comment on sources of experimental error (parallax, non‑perfectly flat mirror, etc.).
Image Formation by a Plane Mirror
When parallel rays from an object strike a plane mirror they are reflected according to the law of reflection. The reflected rays appear to diverge from a point behind the mirror, giving an image with the following characteristics (exactly as stated in the syllabus):
- Virtual – the image cannot be projected onto a screen because the reflected rays do not actually converge.
- Upright – the image has the same orientation as the object.
- Same size as the object.
- Image distance equals object distance: If the object is d cm in front of the mirror, the image appears d cm behind the mirror.
Common Misconceptions
- “The reflected ray is the same physical ray that leaves the mirror.” – In reality the reflected ray is a continuation of the incident ray; the image is formed by the apparent extension of these rays behind the mirror.
- “The normal is the same as the mirror surface.” – The normal is a line perpendicular to the surface, not the surface itself.
- “A plane mirror can produce a real image.” – Plane mirrors only produce virtual images; real images require converging reflected rays (e.g., concave mirrors).
- “The angle of incidence is measured from the mirror surface.” – It is measured from the normal, not from the surface.
Worked Numerical Example (i → r + image distance)
Question: A ray of light strikes a plane mirror at an angle of incidence of 45°. An object is placed 20 cm** in front of the mirror. Determine:
- The angle of reflection.
- The position of the image relative to the mirror.
Solution:
- From the law of reflection, r = i = 45°.
- For a plane mirror, image distance = object distance. Hence the image appears 20 cm behind the mirror, upright and the same size as the object.
Additional Example Questions
1. Law of Reflection
Q: A ray strikes a flat mirror at an angle of incidence of 30°. What is the angle of reflection?
A: By the law of reflection, r = i = 30°.
2. Image Distance
Q: An object is placed 15 cm in front of a plane mirror. State the position of the image.
A: The image is 15 cm behind the mirror, virtual, upright and the same size as the object.
3. Experimental Error
Q: In a lab you measured the following pairs of angles (°): (i = 25, r = 24), (i = 40, r = 41), (i = 55, r = 55). Calculate the average absolute error |i − r|.
A: Errors = 1°, 1°, 0°. Average error = (1 + 1 + 0)/3 = 0.67°.
Summary Table
| Term | Definition | Symbol |
|---|---|---|
| Normal | Line perpendicular to the reflecting surface at the point of incidence | — |
| Angle of Incidence | Angle between the incident ray and the normal | i |
| Angle of Reflection | Angle between the reflected ray and the normal | r |
| Plane‑mirror Image | Virtual, upright, same size as object; image distance = object distance | — |
Quick Revision Checklist
- Normal – perpendicular to surface.
- i = angle between incident ray & normal.
- r = angle between reflected ray & normal.
- Law of reflection: i = r and all three lines lie in one plane.
- Plane‑mirror image: virtual, upright, same size, dimage = dobject.