Lighting fire using the sun and a lens is a neat trick. You can also form a lens using ice. However, in most conditions where you need a fire to survive you don

Lighting fire using the sun and a lens is a neat trick. You can also form a lens using ice. However, in most conditions where you need a fire to survive you don't have sufficient sunlight or sufficiently dry fuel for the method, so I'd recommend learning other methods for lighting fire, in particular the method of always carrying a lighter with you when you leave civilization.