Cambridge IGCSE Physics 0625 – 3.2.1 Reflection of Light
3.2.1 Reflection of Light
Learning Objective
Define and use the terms normal, angle of incidence and angle of reflection in the context of light reflecting from a surface.
Key Definitions
Normal: An imaginary line drawn perpendicular (at 90°) to the 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
When a ray of light strikes a smooth (specular) surface, the angle of incidence is equal to the angle of reflection:
\$ i = r \$
The incident ray, the reflected ray, and the normal all lie in the same plane.
How to Measure the Angles
Draw the surface and mark the point of incidence.
Construct the normal – a line at right angles to the surface at that point.
Measure the angle between the incident ray and the normal; this is \$i\$.
Measure the angle between the reflected ray and the normal; this is \$r\$.
Check that \$i\$ and \$r\$ are equal (within experimental error).
Suggested Diagram
Suggested diagram: A smooth mirror surface with a normal drawn at the point of incidence. The incident ray makes angle \$i\$ with the normal, and the reflected ray makes angle \$r\$ with the normal, showing \$i = r\$.
Summary Table
Term
Definition
Symbol (if any)
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\$
Example Question
A ray of light strikes a flat mirror such that the angle of incidence is \$30^\circ\$. What is the angle of reflection?
Solution: By the law of reflection, \$i = r\$. Therefore \$r = 30^\circ\$.