Notes | Physics

3.2.1 Reflection of Light

Define and use the terms normal, angle of incidence and angle of reflection.

Term	Definition
Normal	An imaginary line that is perpendicular (at a right angle) to the surface at the point of contact. Think of it as a “vertical” line that stands straight up from the surface. 📐
Angle of Incidence ($\theta_i$)	The angle between the incident ray (the incoming light) and the normal. It is measured in degrees or radians. 🔍
Angle of Reflection ($\theta_r$)	The angle between the reflected ray (the outgoing light) and the normal. According to the law of reflection, $\theta_i = \theta_r$. 🌞

Draw the normal line at the point where the light hits the surface.
Measure the angle between the incident ray and the normal – this is $\theta_i$.
After the light bounces, measure the angle between the reflected ray and the normal – this is $\theta_r$.
Check that $\theta_i$ equals $\theta_r$ to confirm the law of reflection.

🔄 Mirror Reflection: Imagine a ball (light) rolling toward a wall (surface). The wall pushes it back at the same angle it came, just like a mirror reflects light.
🌊 Water Surface: Light hitting a calm pond reflects at the same angle, creating a perfect image of the sky.
🪞 Mirror in a Classroom: The normal is the invisible line that would be straight up from the mirror’s surface; the angles of incidence and reflection are measured from this line.
🚗 Driving Analogy: If you drive straight toward a wall and then reverse straight back, the angle you turned to reverse is the same as the angle you approached – that’s the law of reflection.

A ray of light strikes a flat surface at an angle of $30^\circ$ to the normal. What is the angle of reflection? 🤔

Answer: $30^\circ$ – because $\theta_i = \theta_r$.