Activities 10.10 and 10.11 — Converging and Diverging Lenses, and Uses — "Light: Mirrors and Lenses"

1. 14 marksCuriosity Grade 8, Chapter 10, page 164

   Describe Activity 10.10's set-up and its result for the thin glass plate, the convex lens and the concave lens.

   Hint. The same multiple-parallel-beams trick from Activity 10.6, now aimed at lenses.

   The set-up. Fix a thin transparent glass plate upright between two identical books, with paper sheets spread on both books to catch the light. Let multiple parallel beams (from a comb with several openings uncovered) fall on it, and observe. Repeat with a convex lens, then a concave lens, in the same position.

   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.

   Object in the beam's path	Effect on the parallel beams
   Thin glass plate	Passes through unchanged
   Convex lens	Converges
   Concave lens	Diverges

   ✦ The flat glass plate is included as a control, not as an afterthought. Since it has no curved surfaces at all, it shows what happens to the beams when nothing about their shape is bent — nothing — which makes clear that convergence and divergence are caused specifically by the curvature of the convex and concave lenses, and not by merely passing through any transparent material whatsoever.

2. 23 marksCuriosity Grade 8, Chapter 10, page 164

   Why is a convex lens also called a converging lens, and a concave lens a diverging lens?

   Hint. The alternative names describe exactly what Activity 10.10 showed the beams doing.

   A convex lens is also called a converging lens while a concave lens is called a diverging lens.

   The names are simply descriptions of the behaviour observed in Activity 10.10. A convex lens brings parallel beams of light closer together — it converges them — so 'converging lens' is a name for what it does, alongside 'convex lens', which is a name for its shape. Likewise a concave lens spreads parallel beams apart — it diverges them.

   ✦ Having two names for the same object is not redundant; each name is useful in a different context. Since 'convex' and 'concave' describe the shape you can identify by sight or by touch, while 'converging' and 'diverging' describe the behaviour you would only discover by an activity like 10.10, a description built purely on shape (thicker in the middle) predicts the behaviour (converging) precisely because the chapter has established, through experiment, that this particular shape produces that particular effect on a beam of light — the two vocabularies are linked by that result, not independent of it.

3. 33 marksCuriosity Grade 8, Chapter 10, page 164

   How does Activity 10.10's result for the convex lens connect to what Activity 10.6 found for the concave mirror?

   Hint. Two different-shaped devices, converging light the same way.

   Both converge parallel beams of light, despite being entirely different kinds of device — one reflects, the other transmits.

   Activity 10.6: when multiple beams of light fall upon a concave mirror, the multiple reflected beams get closer, that is, they converge.

   Activity 10.10: the convex lens converges the light falling on it.

   ✦ This is why the later activities (10.7 and 10.11) can burn paper using either device. Since both a concave mirror and a convex lens bring a spread-out beam of parallel sunlight together at a small area, both concentrate the Sun's energy enough to ignite paper — one by reflecting the light back to a point, the other by bending the light as it passes through toward a point on the far side. The mechanism differs (reflection versus transmission), but the outcome for a beam of parallel rays is the same word: converge.

4. 43 marksCuriosity Grade 8, Chapter 10, page 165

   Describe Activity 10.11 and its safety precaution. What earlier activity does it mirror, and with what substituted device?

   Hint. Compare the instruction directly with Activity 10.7's.

   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?

   Safety first: Do not look at the Sun directly or through the lens as it may damage your eyes.

   The mirrored activity is 10.7, where a concave mirror's converging reflection was used to burn paper. Activity 10.11 substitutes a convex lens — a device that converges light by transmission rather than reflection — in exactly the same role.

   ✦ The predicted outcome is the same: yes, the paper can be burned, because Activity 10.10 has already established that a convex lens converges parallel beams just as a concave mirror does. Since the underlying cause of the burning in Activity 10.7 was concentrated energy at the point of convergence, and a convex lens produces that same convergence by a different physical route, there is no reason to expect a different result here — only a different mechanism reaching the same destination.

5. 53 marksCuriosity Grade 8, Chapter 10, page 165

   Give four uses of lenses that the chapter names, and identify which of them relies on a lens the chapter says can change its own shape.

   Hint. One of these 'lenses' is not a manufactured object at all.

   The eyeglasses that people wear to help them see clearly 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.

   Use	Type of lens involved
   Eyeglasses	Corrective lenses
   Cameras	Lenses forming an image
   Telescopes	Lenses (or, for reflecting types, a curved mirror)
   Microscopes	Lenses
   The human eye itself	A convex lens that changes shape

   ✦ The eye's lens is the one the chapter singles out as unusual, and for good reason. Since a manufactured lens like a camera's or a pair of spectacles' keeps a fixed shape once made, the eye's convex lens is remarkable precisely because it does not — it changes its shape, and that changing shape is what lets a single eye focus clearly on both a nearby book and a distant object without needing to be swapped for a different lens each time, which is exactly the trade every other device on this list has to make by carrying multiple fixed lenses or adjusting their distance instead.

6. 64 marksCuriosity Grade 8, Chapter 10, page 169

   Describe the Bhāskara II-era astronomers' method with bowls of water, and what the chapter says can and cannot be concluded from it about their knowledge of the laws of reflection.

   Hint. The chapter is careful to distinguish evidence of practice from evidence of stated theory.

   More than 800 years ago, during the time of the great Indian mathematician Bhāskara II, astronomers used shallow bowls of water to observe the stars and planets. By carefully looking at their reflected images through tubes placed at appropriate angles, they could measure the positions of stars and planets in the sky. Even though the laws of reflection are not mentioned in literature, their instruments and methods indicate that they might have understood it in practice!

   What can be concluded, carefully. They used a shallow bowl of still water as a mirror, and viewing tubes set at particular angles to read off the reflected positions of stars and planets — a working method that depends on reflected light behaving predictably, angle for angle.

   ✦ What cannot be concluded, and the chapter is explicit about this: that they wrote down or formally stated the laws of reflection. The laws of reflection are not mentioned in literature — the evidence is entirely in the design of their instruments and methods, which indicate that they might have understood it in practice. Since 'might have understood it in practice' is a hedge and not a claim of documented theory, an accurate answer keeps that same distinction: practical, working knowledge of how reflection behaves is not the same claim as a written statement of the law, and the chapter reports only the former, carefully, as a possibility rather than a certainty.

7. 73 marksCuriosity Grade 8, Chapter 10, pages 164-165

   Read the Snapshot statement: 'a convex lens converges the light beams while a concave lens diverges it.' Which two activities together establish this, and what does each one contribute?

   Hint. One activity gives the naked-eye image; the other gives the beam geometry.

   Activities 10.9 and 10.10 together, contributing two different kinds of evidence for the same fact.

   Activity 10.9 shows what an object seen through each lens looks like — erect and enlarged close up through the convex lens, becoming inverted farther away; always erect and diminished through the concave lens. This is evidence about images, from the viewer's side of the lens.

   Activity 10.10 shows what a beam of parallel light does when it passes through each lens — converging through the convex lens, diverging through the concave lens, while an uncurved glass plate leaves the beams unchanged. This is direct evidence about the light rays themselves.

   ✦ The two activities are not testing the same thing twice; they establish the claim from two independent directions. Since Activity 10.9's image-based evidence and Activity 10.10's beam-based evidence are obtained by completely different methods yet point to the identical conclusion about which lens shape converges and which diverges, the Snapshot statement is not resting on a single observation — it is the conclusion two separate activities were designed to confirm.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 10 (hecu110.pdf), Reprint 2026-27, pages 152-169. Questions are referenced from the NCERT textbook for identification.