Q1.
Repeat Activity 10.7 by putting a convex lens in the path of sunrays in place of a concave mirror (Fig. 10.19). Could you burn the paper?
Answer
Yes. Hold the convex lens so that sunlight passes through it onto the paper, and adjust the distance until the patch of light shrinks to the smallest, brightest spot. Held steady there for a minute or two, the paper browns, smokes and can catch fire — exactly as it did with the concave mirror.
Why the lens does the job just as well: the Sun's rays arrive parallel. A convex lens brings a parallel beam together at its focus, so all the light energy that fell on the whole face of the lens is delivered to a spot a few millimetres across. The energy arriving there each second far exceeds what the paper can carry away to the air, so its temperature rises until it ignites. The mirror does this by reflection and the lens by refraction, but the physical reason — concentration of light energy — is identical.
Safety first: do not look at the Sun directly or through the lens; it may damage your eyes. Work under adult supervision and keep water nearby.