3.2.1 Reflection of Light
Objective
Define and use the terms normal, angle of incidence and angle of reflection.
Key Terms
| 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, \$\thetai = \thetar\$. 🌞 |
How to Measure Angles
- 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 \$\thetai\$ equals \$\thetar\$ to confirm the law of reflection.
Analogies & Examples
- 🔄 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.
Quick Practice
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 \$\thetai = \thetar\$.