A magnifying glass is great for starting fires. It works well as long as the tinder is small and dry like shaved wood, crunched leaves, etc.
I keep a magnifying glass in each one of my kits, as well as traditional methods to start a fire like a lighter, matches, magnesium fire-starter and Vaseline cotton-ball tinder.
A magnifying glass will save on using matches for cloudy days or nighttime while during sunshine you can easily start a fire for free.
Using a magnifying glass will typically get a tinder pile going in less than a minute in full sunlight, sometimes just seconds… Your time will vary depending on the time of day, the angle of the sun, the efficiency of the magnifying lens, and the tinder being used.
The quality of the glass (which is often plastic) makes a significant difference as to the ability to focus a tiny bright spot onto the tinder so as to ignite more quickly. Plastic lenses will work, however many of the cheap ones are simply too poorly manufactured to focus a concentrated beam from the sun. Glass or quality acrylic will be better.
The diameter of the typical size magnifying lens is not extremely important in my experience, although the larger diameter (greater than just a few inches) will pick up more available light and is quicker and easier to start a fire.
A word of caution (besides the obvious when making fire) is to avoid looking at the concentrated point of light, which could damage your eyes. Sunglasses are in order.
The science behind lighting a fire with a magnifying glass can be found in the form of photons. Photons are the particles that carry visible light from the sun to the earth. They also contain energy in the form of heat. Through the use of a magnifying glass, the path of these photons are narrowed to a highly localized area (the dot of light that passes through the convex shaped lens). This results in a concentration of heat that can reach incredibly high temperatures. If a high enough temperature is reached (somewhere around 450 degrees F), the tinder or kindling will smolder and begin to burn.
You could get a magnifying glass like this one, remove it from the frame, and you will be left with a 2 1/2 inch glass lens which won’t scratch like acrylic and you can drop it into your kit.
Most all varieties that are available are not expensive at all. No excuses not to have several… You start paying more money for the photo-quality type lenses, which is irrelevant in this survival scenario.