Light: Mirrors and Lenses — Class 8 Science (Curiosity)
"These are not plane mirrors. These are spherical mirrors. When the mirror is curved inward or outward, your image looks different in them!" — a science-centre guide, Curiosity, Grade 8, page 153
1. About the Chapter
This is Chapter 10 of Curiosity (pages 152–169, Reprint 2026-27). Meena visits a science centre and sees her face enlarged in one curved mirror, upside down in another, and tiny in a third — nothing like the plane mirror she knows from Grade 7. The whole chapter grows out of explaining her confusion.
| Section | Question |
|---|---|
| 10.1 | What are spherical mirrors? |
| 10.2 | What are the characteristics of images formed by spherical mirrors? |
| 10.3 | What are the laws of reflection? |
| 10.4 | What is a lens? |
Everything here comes from eleven hands-on activities — a spoon, a side-view comparison, an object moved toward and away from two mirrors, a torch beam traced with a protractor, a beam bent out of its own plane, multiple parallel beams through mirrors and lenses, sunlight focused to burn paper (twice), and a drop of water standing in for a lens.
What this chapter is not. There is no focal point, no centre of curvature, no pole, and no real-versus-virtual image classification — images are described only as erect or inverted, enlarged or diminished. There is no section on refraction, no human-eye anatomy beyond one sentence, and no periscope or kaleidoscope. All of that is later syllabus.
2. What Are Spherical Mirrors?
Activity 10.1 — the spoon
Hold a shiny metallic spoon's curved surface close to your face.
When you looked at the inner side of the spoon which is curved inwards, you must have observed that the image was inverted. When you looked at the outer side of the spoon which bulges outwards, the image of your face was erect but smaller in size.
| Side of the spoon | Curve | Image |
|---|---|---|
| Inner | Inward | Inverted |
| Outer | Outward | Erect, smaller |
One ordinary object gives you both new kinds of mirror at once.
Definitions
Spherical mirrors are a common type of curved mirrors which are shaped like a part of a hollow glass sphere.
A spherical mirror, which has a reflecting surface that curves inwards, is called a concave mirror. A reflecting surface that curves outwards is a convex mirror. The outline of the surface of the mirror is circular.
How they are actually made. Spherical mirrors are not made by slicing a hollow glass sphere. Instead, they are created by grinding and polishing a flat glass piece into a curved surface. Coat the outer curved surface → concave mirror. Coat the inner curved surface → convex mirror. "Part of a hollow sphere" describes the shape; it is not how the mirror is manufactured.
Activity 10.2 — telling them apart from the side
Place both mirrors reflecting-side-up on a table and view them from the side, eye level with the rim. A concave mirror's surface dips down in the middle; a convex mirror's bulges up. Looking straight down would not show this at all.
3. Characteristics of Images in Spherical Mirrors
Activity 10.3
Object 3–4 cm from both mirrors, then moved farther away.
In the concave mirror, when the object is placed close to the mirror, the image is erect but larger than the object in size, that is, enlarged. However, when the object is moved farther away, the image becomes inverted. Initially, the image is enlarged in size and then keeps getting smaller.
In case of a convex mirror, the image is always erect and smaller than the object, that is diminished. However, the size of the image decreases slightly as the object is moved away.
| Concave | Convex | |
|---|---|---|
| Close | Erect, enlarged | Erect, diminished |
| Farther | Inverted (first enlarged, then shrinking) | Always erect, diminished |
Lateral inversion of the image is seen in all three types of mirrors.
Lateral inversion (left-right) and erect/inverted (top-bottom) are two separate properties — track both.
We can also identify whether a mirror is plane, or concave, or convex by looking at the images of an object formed in them!
Real-world uses
Concave — close-up, enlarging: the reflectors of torches, headlights of cars and scooters; a dental mirror ... provides an enlarged view of teeth when held close; the main mirror of a reflecting telescope.
Convex — always diminished, wide field: side-view mirrors on vehicles ... always form an erect image of the traffic behind and smaller than the actual vehicles... it provides a much wider area of the road behind; road-safety mirrors at bends and intersections; store surveillance mirrors.
4. The Laws of Reflection
Activity 10.4 — terms and the first law
A single-slit torch beam onto a plane mirror, drawn on paper.
| Term | Definition |
|---|---|
| Incident ray | The ray of light that falls on the mirror |
| Reflected ray | The ray of light that comes back from the mirror |
| Normal | A line at 90° to the mirror at the point of incidence |
| Angle of incidence (i) | Between the normal and the incident ray |
| Angle of reflection (r) | Between the normal and the reflected ray |
If done carefully, the experiment shows that the angle of incidence (i) is equal to the angle of reflection (r). This is a law of reflection.
Repeated at several angles — not just once — and confirmed once more at 0°, along the normal, where both the angles would be zero.
Activity 10.5 — the second law
A beam reflected onto paper extending off the table; bend that extended part down.
The reflected beam disappears when the sheet is bent but reappears when it is flattened again. This shows that the reflected beam lies in the same plane as that of the incident beam.
The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.
Even though the directions of incident rays are different, they fall at the same point on the mirror, and thus, the directions of normal are the same — so bending the paper cannot be blamed on a shifted normal; the law itself is what's being tested.
Activity 10.6 — parallel beams meet a curve
The laws of reflection are valid for all kinds of mirrors — plane and spherical.
Multiple parallel beams onto plane, concave and convex mirrors:
| Mirror | Reflected beams |
|---|---|
| Plane | Stay parallel |
| Concave | Converge |
| Convex | Diverge |
No contradiction with the laws. Each ray still obeys i = r at its own point — but a curved surface gives each point a differently angled normal, so identical obedience to the law, point by point, bends the whole beam in or out.
Activity 10.7 — burning paper
Safety first: Do not look towards the Sun or into the mirror reflecting the Sun. Focus the reflected light only on a piece of paper.
The bright spot is formed on the paper because light from the Sun, after reflection from the mirror, gets concentrated on this point. This produces sufficient heat at this point which can ignite the paper.
Solar concentrators scale this up: the concentrated sunlight is used to heat a liquid to produce steam ... for large scale cooking or for solar furnaces. Solar furnaces are even used for melting steel!
5. What Is a Lens?
Activity 10.8 — the water drop
Oil a glass strip so a water drop stays rounded; view text beneath it.
The surface of the water drop is curved outside. The letters under the water drop look different — they might appear larger than the letters nearby! ... This curved drop of water is acting like a simple lens.
A lens is a piece of transparent material ... which has curved surfaces. Thicker at the middle → convex lens. Thicker at the edges → concave lens.
Unlike mirrors, lenses allow light to pass through them, and we see things through a lens rather than in a lens.
Activity 10.9 — image characteristics through lenses
When an object is placed behind a convex lens at a small distance from it ... the object appears erect and enlarged in size. As the distance ... increases, the object appears inverted. It is initially enlarged in size and then diminishes in size.
An object placed behind a concave lens ... always appears erect and diminished in size. Its size changes, as its distance from the lens increases.
Notice the pattern is identical to the mirrors — convex lens behaves like the concave mirror (distance-dependent); concave lens behaves like the convex mirror (always diminished, never inverted).
Activities 10.10 and 10.11 — converging, diverging, burning again
The light beam passes through the thin glass plate as it is. The convex lens converges the light falling on it while the concave lens diverges the light. A convex lens is also called a converging lens while a concave lens is called a diverging lens.
Repeat Activity 10.7 by putting a convex lens in the path of sunrays in place of a concave mirror. Could you burn the paper? Yes — the same convergence, by transmission instead of reflection.
Uses
The eyeglasses that people wear ... are lenses. Cameras, telescopes, and microscopes all use lenses to work. Even our eye has a convex lens inside it. It is quite an amazing lens that can change its shape, which is what allows us to read a book or see something far away.
Our scientific heritage. More than 800 years ago, during the time of ... Bhāskara II, astronomers used shallow bowls of water to observe the stars and planets ... Even though the laws of reflection are not mentioned in literature, their instruments and methods indicate that they might have understood it in practice! Practical skill, honestly distinguished from a documented theory.
6. Reading the Chapter's Own Matching Exercises
Three of the chapter's exercises are photographs you match by comparing sizes directly, not by reading a printed order:
- Fig. 10.23 (pen cap, three mirrors): same size → plane; larger → concave; smaller → convex.
- Fig. 10.24 (pen cap, convex lens / concave lens / flat glass): larger → convex lens; smaller → concave lens; unchanged → flat glass (no curve, nothing to converge or diverge).
- Fig. 10.25 (graph sheet, three mirrors): grid squares magnified → concave; unchanged → plane; compressed → convex.
- Fig. 10.27 (O–M–I diagrams): both images erect in (a) and (b); same height → plane; taller → concave (never a convex mirror, which is always diminished).
7. The Traps
Bringing in focal length, centre of curvature, or real/virtual images. None of this vocabulary appears in the chapter — everything is erect/inverted, enlarged/diminished.
Measuring an angle from the mirror instead of the normal. The laws are defined from the normal; the angle from the mirror surface is 90° minus that.
Saying a convex mirror ever inverts, or a concave mirror is always enlarged. Convex: always erect, diminished. Concave: erect+enlarged only close up; inverted farther away.
Confusing lateral inversion with erect/inverted. Lateral inversion is left-right, present in all mirrors always; erect/inverted is top-bottom, and only the concave mirror (and convex lens) changes it.
Saying "in" a lens. Light passes through a lens; you look into a mirror.
Treating the Bhāskara II heritage box as proof of a written law. The chapter says the laws are not mentioned in literature — only that practice might show understanding.
Importing refraction, eye anatomy/defects, or detailed optical instruments. Not in this chapter.
8. What to Carry Forward
- Concave curves inward; convex curves outward — for both mirrors and lenses.
- Concave mirror / convex lens: erect+enlarged close, inverted (first enlarged, then diminishing) farther.
- Convex mirror / concave lens: always erect and diminished, at every distance.
- Lateral inversion occurs in every mirror, independent of erect/inverted.
- Two laws of reflection: i = r (from the normal); incident ray, normal, reflected ray coplanar. Valid for all mirrors, plane or spherical.
- Concave mirrors and convex lenses converge parallel beams; convex mirrors and concave lenses diverge them.
- Concentrated light generates real heat — Activities 10.7 and 10.11 both ignite paper; solar concentrators scale the same idea up.
- We see through a lens, into a mirror.
