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Chapter 10 — Light: Mirrors and Lenses
Table of Contents
10.1 What Are Spherical Mirrors?
A spherical mirror is shaped like a part of a hollow glass sphere — its reflecting surface is curved!
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Fig 10.1a — Inner curved surface (concave) → Inverted image
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Fig 10.1b — Outer curved surface (convex) → Erect, smaller image
- Inner side of spoon (curved inward) → image is inverted 🙃
- Outer side of spoon (curves outward) → image is erect but smaller
🪞 Two Types of Spherical Mirrors —
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Fig 10.2a — Concave mirror (curves inward)
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Fig 10.3a — Convex mirror (curves outward)
Concave Mirror
Reflecting surface curves inward (like a bowl/cave). Also called converging mirror.
Convex Mirror
Reflecting surface curves outward (like back of spoon). Also called diverging mirror.
👁️ How to tell them apart? Look from the side!
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Fig 10.4 — Side view: Concave (left, curves in) vs Convex (right, curves out)
🔑 Key Definitions
Concave mirror = reflecting surface curves inward | Convex mirror = reflecting surface curves outward. Both are spherical mirrors — shaped like part of a hollow sphere.
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10.2 Images in Spherical Mirrors
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Fig 10.5a — Object at small distance (3–4 cm)
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Fig 10.5b — Object at large distance
| Mirror Type | Object Close | Object Far | Always? |
|---|---|---|---|
| 🪞 Plane | Erect, same size | Erect, same size | Always same size |
| 🥣 Concave | Erect, enlarged | Inverted, getting smaller | Size changes! |
| 🫧 Convex | Erect, diminished | Erect, smaller still | Always erect & smaller |
🔑 Key Rule
Concave mirror → image can be enlarged OR inverted depending on distance.
Convex mirror → image is ALWAYS erect and diminished (smaller).
Lateral inversion (left-right flip) is seen in ALL three types of mirrors.
🔦 Concave Mirror Uses
Torch & car headlight reflectors, dentist's mirror (enlarged view of teeth), telescope mirrors, solar concentrators!
🚗 Convex Mirror Uses
Side-view mirrors in vehicles (wider view), road safety mirrors at bends, surveillance mirrors in shops!
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Fig 10.7a — Side-view mirror (convex) on bike
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Fig 10.7c — Surveillance mirror (convex) in store
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10.3 Laws of Reflection
💡 Key Terms —
- Incident Ray — ray of light that falls on the mirror
- Reflected Ray — ray that bounces back from the mirror
- Normal — line perpendicular (90°) to mirror at point of incidence
- Angle of Incidence (i) — angle between incident ray & normal
- Angle of Reflection (r) — angle between reflected ray & normal
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Fig 10.9 — Incident ray, Normal, Reflected ray
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Fig 10.9d — Measuring angle of incidence & reflection
🔑 TWO Laws of Reflection
Law 1: Angle of incidence (i) = Angle of reflection (r) → ∠i = ∠r
Law 2: The incident ray, the normal at the point of incidence, and the reflected ray — all lie in the same plane.
These laws are valid for ALL mirrors — plane, concave, and convex!
🔬 Second Law Experiment (Bent Paper):
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Fig 10.10a — Flat paper: reflected beam visible ✅
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Fig 10.10b — Bent paper: reflected beam disappears ❌
When paper is flat → incident ray, normal, and reflected ray are all in same plane → reflected beam visible. When paper is bent → a new plane is created → reflected beam disappears!
⚡ What happens with multiple parallel beams?
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Plane mirror → beams stay parallel
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Concave mirror → beams CONVERGE
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Convex mirror → beams DIVERGE
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10.4 What Is a Lens?
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Fig 10.13 — Water drop acts like a convex lens!
A drop of water on glass magnifies text beneath it — it acts like a simple lens!
- A lens = transparent material (glass/plastic) with curved surfaces
- Unlike mirrors, light passes through a lens
- Lenses can be convex or concave, just like mirrors
Convex Lens
Thicker at the middle, thinner at edges. Also called converging lens. e.g. Magnifying glass.
Concave Lens
Thicker at the edges, thinner at middle. Also called diverging lens. e.g. Spectacles for myopia.
🔑 Remember!
Mirrors REFLECT light (we see image in the mirror). Lenses REFRACT light (we see object through the lens). Both can form enlarged, diminished, erect, or inverted images!
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10.5 Images Formed by Lenses
🔎 Convex Lens —
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Fig 10.17a — Object close: Erect & enlarged ✅
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Fig 10.17b — Object far: Inverted (then smaller)
🔭 Concave Lens —
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Fig 10.17c — Concave lens: Always erect & diminished
| Lens | Object Close | Object Far |
|---|---|---|
| 🔎 Convex | Erect, enlarged | Inverted |
| 🔭 Concave | Erect, diminished | Erect, smaller |
Concave lens ALWAYS forms erect & diminished image — no matter the distance!
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10.6 Converging vs Diverging
💡 Light through Lenses:
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Flat glass plate — parallel beam stays parallel
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Convex lens — beam CONVERGES (comes together)
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Concave lens — beam DIVERGES (spreads out)
| Type | Effect on Light | Other name | Can burn paper? |
|---|---|---|---|
| 🥣 Concave mirror | Converges | Converging mirror | ✅ Yes (sunlight focus) |
| 🫧 Convex mirror | Diverges | Diverging mirror | ❌ No |
| 🔎 Convex lens | Converges | Converging lens | ✅ Yes (sunlight focus) |
| 🔭 Concave lens | Diverges | Diverging lens | ❌ No |
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Fig 10.12 — Concave mirror converging sunlight → burns paper!
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Fig 10.19 — Convex lens also converges sunlight → burns paper!
Safety First! Never look at the Sun directly or through a lens — it can permanently damage your eyes! Always work under adult supervision.
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10.7 Uses of Mirrors & Lenses
🪞 Concave Mirror Uses —
- 🔦 Torch & headlights — reflector behind bulb focuses light into beam
- 🦷 Dentist's mirror — enlarged view of teeth when held close
- 🔭 Reflecting telescope — large concave mirror collects starlight
- ☀️ Solar concentrators/furnaces — focuses sunlight to generate heat
🫧 Convex Mirror Uses —
- 🚗 Side-view mirrors — wider field of view → "Objects closer than they appear"
- 🛤️ Road safety mirrors — at sharp bends to prevent collisions
- 🏬 Surveillance mirrors — in shops to monitor large areas
🔎 Lens Uses —
- 👓 Eyeglasses — help people see clearly
- 📷 Camera & smartphone — lenses capture images
- 🔭 Telescope & microscope — magnify distant/tiny objects
- 👁️ Human eye — has a convex lens inside that changes shape!
- 🔎 Magnifying glass — convex lens, enlarges small print
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Fig 10.26 — Woman walking toward concave mirror — image inverts!
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Fig 10.29 — Solar cooker uses concave mirrors to cook food!
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Fig 10.20b — Smartphone camera — multiple lenses inside!
Our Scientific Heritage!
800+ years ago, during the time of mathematician Bhāskara II, Indian astronomers used shallow bowls of water as mirrors to observe stars & planets! Through reflected images in these water bowls, they could measure positions of stars — showing they understood reflection in practice!
800+ years ago, during the time of mathematician Bhāskara II, Indian astronomers used shallow bowls of water as mirrors to observe stars & planets! Through reflected images in these water bowls, they could measure positions of stars — showing they understood reflection in practice!
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Quick Summary
- Spherical mirrors: Concave (curves in) and Convex (curves out)
- Concave mirror → image can be enlarged OR inverted (depends on distance)
- Convex mirror → image ALWAYS erect and diminished
- Law 1: ∠i = ∠r (angle of incidence = angle of reflection)
- Law 2: Incident ray, normal, reflected ray — all in same plane
- Laws of reflection valid for ALL mirrors
- Concave mirror → converges light | Convex mirror → diverges light
- Lens = transparent material with curved surfaces, light passes through
- Convex lens (converging) → can enlarge or invert depending on distance
- Concave lens (diverging) → always erect and diminished
- Lateral inversion seen in ALL three types of mirrors
🪞 Mirror & Lens Cheat Sheet
Law of Reflection∠i = ∠r
Concave mirrorConverges light → can enlarge/invert
Convex mirrorDiverges → always erect & diminished
Convex lensConverging → can enlarge/invert
Concave lensDiverging → always erect & diminished
Side-view mirrorConvex → wider view