The angle of incidence equals the angle of reflection: θᵢ = θᵣ. Both angles are measured from the normal to the surface.
Image is virtual, upright, same size, and located the same distance behind the mirror as the object is in front. Laterally inverted (left-right swap).
Concave (converging): f = R/2 (positive). Convex (diverging): f = −R/2 (negative). Mirror equation: 1/f = 1/dᵢ + 1/dₒ. Magnification: m = −dₒ/dᵢ. Real images: dₒ > 0 (inverted). Virtual images: dₒ < 0 (upright).
💡θᵢ = θᵣ. Mirror equation: 1/f = 1/dᵢ + 1/dₒ. m = −dₒ/dᵢ. Sign conventions are critical.
📋 Key Formulas
📝 Worked Example 1
Q: Object 30 cm from concave mirror, f = 20 cm. Image location and type?
A: 1/20 = 1/30 + 1/dₒ → 1/dₒ = 1/20 − 1/30 = 1/60 → dₒ = 60 cm. Real, inverted, m = −60/30 = −2 (enlarged).
📝 Worked Example 2
Q: Object 10 cm from concave mirror, f = 20 cm. Image?
A: 1/20 = 1/10 + 1/dₒ → 1/dₒ = 1/20 − 1/10 = −1/20 → dₒ = −20 cm. Virtual, upright, m = 2.
📝 Worked Example 3
Q: Convex mirror f = −15 cm, object 30 cm away. Image?
A: 1/(−15) = 1/30 + 1/dₒ → dₒ = −10 cm. Virtual, upright, m = 1/3 (diminished).
🧠Follow sign conventions strictly.
🧠Draw ray diagrams to verify.
🧠Convex mirrors always give virtual, diminished images.
⚠️Wrong sign for f in convex mirrors.
⚠️Forgetting the negative in m = −dₒ/dᵢ.
⚠️Not checking if image is real or virtual.
🎯 Try This Yourself
Open and read all sections to complete this module