What is Reflection?
Reflection is when a light ray hits a surface and bounces back, just like a ball hitting a wall.
Law of Reflection
The law says: the angle of incidence (the angle between the incoming ray and an imaginary line called the normal) equals the angle of reflection (the angle between the reflected ray and the normal). The normal is an invisible line drawn perpendicular (at a right angle) to the surface at the point where the ray meets it.
In plain words, whatever way the ray comes in, it leaves at the same tilt on the other side of that invisible line.
Ray Diagram Basics
To draw a ray diagram, follow these steps:
- Draw the reflecting surface (usually a straight line for a plane mirror).
- Mark the point where the ray hits – the point of incidence.
- Draw the normal at that point – a short dashed line perpendicular to the mirror.
- From the source, draw the incoming ray making the angle of incidence with the normal.
- From the point of incidence, draw the reflected ray on the other side of the normal, making the same angle.
Remember: the incident ray, reflected ray, and the normal all lie in the same plane (a flat sheet of paper).
Worked Example
Suppose a ray from a candle hits a flat mirror at a point where the normal makes a 30° angle with the incoming ray. What does the diagram look like?
- Draw a horizontal line for the mirror.
- Mark a point “P” on the line – that’s the point of incidence.
- Draw a short vertical dashed line through P – this is the normal.
- From the candle (say left side), draw a line to P that makes a 30° angle with the normal. That’s the incident ray.
- Now draw another line starting at P on the opposite side of the normal, also at 30°. That’s the reflected ray.
Notice the two rays are mirror images of each other across the normal. If you measure the angles, they’re both 30° – exactly what the law predicts.
Key Points Summary
| Concept | What to Remember |
|---|---|
| Normal | Imaginary line perpendicular to the surface at the point of incidence. |
| Angle of Incidence | Angle between incoming ray and normal. |
| Angle of Reflection | Angle between reflected ray and normal; always equals angle of incidence. |
| Ray Diagram Steps | Surface → point of incidence → normal → draw incident ray → draw reflected ray with equal angle. |
| Plane Mirror | Flat reflecting surface; laws work exactly as stated. |
Quick tip: when you’re unsure, draw the normal first. It’s the anchor that keeps the angles equal.
📝 Likely Exam Questions
- State the law of reflection.
Answer: The angle of incidence equals the angle of reflection, measured with respect to the normal. - What is the normal in a ray diagram and why is it needed?
Answer: The normal is an imaginary line drawn perpendicular to the reflecting surface at the point of incidence. It provides a reference line to measure both the incident and reflected angles. - Draw a ray diagram for a ray incident at 45° on a plane mirror.
Answer: (Students sketch a horizontal mirror, a vertical normal at the point of incidence, an incoming ray making 45° with the normal, and a reflected ray on the other side also at 45°.) - If the angle of incidence is 20°, what is the angle of reflection?
Answer: 20°, because the two angles are equal. - Explain why the incident ray, reflected ray, and normal always lie in the same plane.
Answer: Because the mirror surface is flat, any line drawn perpendicular to it (the normal) and any rays interacting with that point will be confined to a single flat surface, i.e., a plane.