Use simple constructions, measurements and calculations for reflection by plane mirrors

Key Concepts

When light rays hit a smooth surface, they bounce back instead of passing through. This is called reflection. In a plane mirror (a flat mirror), the reflected ray behaves in a very predictable way.

• Incident ray (the incoming ray) is denoted by \$i\$.
• Reflected ray (the outgoing ray) is denoted by \$r\$.
• Both rays make the same angle with the normal (an imaginary line perpendicular to the surface).

Reflection Law

🔍 The law of reflection states that the angle of incidence equals the angle of reflection:

\$\thetai = \thetar\$

Both angles are measured from the normal. This simple rule lets us predict where a reflected ray will go.

Simple Construction

✏️ Build a basic experiment to see reflection in action:

1. Place a flat mirror on a table.
2. Hold a flashlight or a laser pointer at a known angle to the mirror.
3. Mark the incident ray, the normal, and the reflected ray on a sheet of paper.
4. Use a protractor to measure the angles.

??

If your measurements show that the two angles are equal, you’ve confirmed the law of reflection!

Measurements & Calculations

📏 In a plane mirror, the image appears to be the same distance behind the mirror as the object is in front of it.

Let \$do\$ be the object distance and \$di\$ be the image distance:

\$di = do\$

Example: If a book is 30 cm from the mirror, the image appears 30 cm behind the mirror.

Exam Tips

• 📚 Diagram accuracy matters: label all rays, normals, and angles clearly.
• 🧮 Use the law of reflection to find unknown angles.
• 🔢 Remember \$di = do\$ for plane mirrors when calculating image positions.
• 📝 Show all steps in calculations; partial credit is awarded for clear reasoning.
• 💡 Analogy trick: Think of a hallway mirror – the image is just behind the mirror, not in front.