NCERT Solutions Curiosity Chapter 10 .3 What Are the Laws of Reflection? — Activity 10.7: Let us explore

Book page 16110 Updated on2026-09-05

Q1.

Adjust the distance of the paper until you get a sharp bright spot on it as shown in Fig. 10.12.

Answer

Move the paper slowly towards and away from the mirror. The patch of light first shrinks, becomes a small, very bright, sharply-edged spot, and then starts to widen again. Stop at the position where the spot is smallest and brightest.

Why there is one best distance: the Sun is so far away that its rays reach the mirror effectively parallel. A concave mirror brings a parallel beam together at one particular point in front of it, its focus. Hold the paper nearer than that and the rays have not yet met; hold it farther and they have already met and are spreading again. Only at the focus is all the reflected light squeezed into a single small spot.

Tip: the distance you have just found is the focal length of your mirror. Measure it from the mirror to the paper and write it in your notebook — deeper mirrors have shorter focal lengths.

Q2.

Hold the mirror and the sheet of paper steady for a few minutes. Does the paper start to burn producing smoke?

Answer

Yes. Held steady, the paper at the bright spot first darkens to brown, then gives off a thin curl of smoke, and can finally catch fire.

Why the paper burns: the mirror collects the sunlight falling on its whole face and pours it onto a spot a few millimetres wide. The energy arriving on that spot each second is now many times what the same paper would receive lying in the open, while the paper can still only lose heat to the air as slowly as before. So heat piles up, the temperature of that tiny patch rises past the temperature at which paper ignites, and it burns. A plane mirror of the same size cannot do this — it spreads the same energy over just as large an area as before, so nothing gets hotter.

Safety first: do this only under a teacher's or an adult's supervision, never look towards the Sun or into the mirror, focus the light only on paper, and keep some water at hand.

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