Final answer:
Glasses with converging lenses, which are thicker at the center, can focus sunlight to start a fire. The more powerful the lens, the shorter its focal length, which causes the light to cross closer to the lens, potentially igniting paper.
Step-by-step explanation:
To start a fire using a lens to focus the sun's rays, glasses with converging lenses would be helpful. A converging lens is thicker at the center and can focus parallel light rays from the sun to a point, known as the focal point. The concentrated sunlight can generate enough heat to ignite paper. When attempting to do this, please ensure safety by being vigilant as the paper may start to smoke and burn from the concentrated light energy.
Determining focal lengths of various lenses can help you find the best lens for starting a fire. A more powerful lens will have a shorter focal length, causing the rays to cross closer to the lens and create a brighter and hotter spot that could start a fire more easily. It is important to note, while conducting this experiment, that the bright spot on the paper is not an image of the Sun, but rather a concentration of sunlight.