Under the strong sunlight, the light at the focus of the convex lens will be particularly violent, and the focus will zoom or catch fire.

The light collecting ability of a lens is the square of the ratio of its aperture to focal length (relative aperture), so a lens with a large aperture is beneficial to improve the light collecting ability. On the other hand, if the protrusion is shallow, the focal length will be large, which is not conducive to light collection. This is because the ice has cold air, and the cold air drops, so the focal length should be slightly larger to reduce the cold effect of the ice lens and make the fire extinguishing easy to succeed.

Extended data

In order to see the tiny object or the details of the object clearly, it is necessary to move the object closer to the eye, which can increase the viewing angle and form a larger real image on the retina. But when the object is too close to the eye, it is not clear. In other words, to be good at observation, we should not only make the object have a large enough opening angle to the eyes, but also take a suitable distance.

Obviously, for the eyes, these two requirements are mutually restrictive. If you put a convex lens in front of your eyes, this problem can be solved. The convex lens is the simplest magnifying glass and a simple optical instrument to help the eyes observe tiny objects or details.

Baidu encyclopedia-magnifying glass

Baidu Encyclopedia-Focus on Fire