Activities 10.6 and 10.7 — Converging, Diverging, and Burning Paper — "Light: Mirrors and Lenses"

1. 14 marksCuriosity Grade 8, Chapter 10, page 160

   Describe Activity 10.6 and its results for plane, concave and convex mirrors.

   Hint. This time, many beams go in together, not just one.

   The set-up. Use the same apparatus as Activity 10.4, but leave many openings of the comb uncovered instead of just one slit, producing several parallel beams of light at once. Let these fall on a plane mirror, a concave mirror and a convex mirror, one at a time, and observe the reflected beams.

   When multiple parallel beams of light fall upon a plane mirror, the multiple reflected beams are also parallel. However, when multiple beams of light fall upon a concave mirror, the multiple reflected beams get closer, that is, they converge. Whereas, in the case of a convex mirror, the multiple reflected beams spread, that is, they diverge.

   Mirror	What the reflected beams do
   Plane	Stay parallel
   Concave	Converge
   Convex	Diverge

   ✦ A single ray could never show this — only many rays together can. One beam obeys i = r wherever it lands and gives you no sense of whether nearby beams are drawing together or spreading apart. Since convergence and divergence are properties of a whole beam of parallel rays, not of any one ray by itself, Activity 10.6 has to switch from a single slit to many openings to reveal them at all.

2. 23 marksCuriosity Grade 8, Chapter 10, page 160

   The chapter says each individual ray in Activity 10.6 still obeys the laws of reflection, yet the beams as a whole converge or diverge. How can both be true?

   Hint. This is the same resolution as the previous group's question, applied now to what you can actually watch happen.

   In the case of spherical mirrors, even though each ray of light follows the laws of reflection, the curved surface of spherical mirrors causes the parallel beam of rays to either converge (concave) or diverge (convex) on reflection depending on the shape of the mirror.

   Both are true because they describe different things. 'Each ray obeys i = r' is a statement about one ray at one point, measured against the normal at that exact spot. 'The beams converge or diverge' is a statement about many rays together, striking many different points where the normal points in a different direction each time.

   ✦ A curved mirror gives each parallel ray a differently angled normal to obey the law against, so reflecting correctly at every single point still bends the overall beam inward or outward. Since the law never stops applying, convergence and divergence are not violations of the laws of reflection — they are the consequence of applying the same unbroken law across a surface whose normal direction keeps changing from point to point.

3. 34 marksCuriosity Grade 8, Chapter 10, page 161

   Describe Activity 10.7 and its safety precautions. Explain why a bright spot forms and why the paper eventually smokes.

   Hint. The concave mirror's convergence, put to a striking demonstration.

   The activity. Hold a concave mirror with its reflecting surface facing the Sun, and direct the reflected sunlight onto a sheet of thin paper. Adjust the paper's distance until a sharp bright spot appears. Hold both steady for a few minutes.

   Safety first: Always perform this activity under the supervision of a teacher or an adult. Do not look towards the Sun or into the mirror reflecting the Sun. Focus the reflected light only on a piece of paper, not towards anyone's face or eyes.

   Why the bright spot forms, and why it burns:

   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.

   ✦ This is Activity 10.6's convergence made visible with real consequences. Sunlight reaching the whole mirror is spread over its entire curved area, carrying only a modest amount of energy per unit area; since a concave mirror converges parallel rays to (nearly) one small point, all of that spread-out energy is funnelled onto a tiny spot instead, and the energy per unit area there is enormously higher than it was at the mirror's surface — high enough, sustained for a few minutes, to set paper alight.

4. 43 marksCuriosity Grade 8, Chapter 10, page 161

   What are solar concentrators? Describe two uses the chapter gives.

   Hint. The burning-paper activity is a small demonstration of a real technology.

   Devices which concentrate sunlight into a small area, using mirrors and lenses, are called solar concentrators. The concentrated sunlight is used to heat a liquid to produce steam which can be used to generate electricity or for providing heat for various purposes, such as large scale cooking or for solar furnaces. Solar furnaces are even used for melting steel!

   Two uses named in the chapter:

   1. Generating electricity — concentrated sunlight heats a liquid to steam, which can drive a generator.
   2. Large-scale cooking, and solar furnaces hot enough to melt steel.

   ✦ Activity 10.7 is a solar concentrator at the scale of a single sheet of paper. A concave mirror focuses the Sun's spread-out energy onto one small spot, and enough heat builds up there to ignite paper — so the same convergence, scaled up with a much larger mirror (or many mirrors aimed at one point), can concentrate enough heat to boil water into steam or even melt steel. Since the underlying physics is identical, the paper-burning demonstration is not a curiosity separate from solar technology — it is the same principle, small enough and safe enough to try in a classroom.

5. 53 marksCuriosity Grade 8, Chapter 10, pages 160-161

   Why does the concave mirror in Activity 10.7 have to be held with its reflecting surface facing the Sun, rather than facing the paper directly at a fixed distance?

   Hint. The paper's distance from the mirror is adjusted, not fixed in advance — ask why that adjustment matters.

   Because the bright spot only forms at one particular distance from the mirror, and that distance has to be found by adjustment, not guessed in advance.

   Adjust the distance of the paper until you get a sharp bright spot on it.

   A concave mirror converges parallel rays of sunlight toward a particular point. Hold the paper too close or too far from that point and the light lands on the paper as a larger, dimmer patch rather than a small, intensely bright spot — since the rays have not yet come together, or have already started to spread apart again past the point of convergence.

   ✦ This is exactly why the instruction is to adjust rather than to fix a distance. Because the point where the rays converge most tightly depends on the mirror's own curvature, no single distance would work for every mirror — the activity has you find that distance experimentally, by moving the paper until the spot is as small and bright as it can be, which is also the point where the concentrated heat is greatest and ignition is most likely.

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.