burning glass
large convex lens that can concentrate the sun's rays onto a small area, heating up the area and thus resulting in ignition of the exposed surface
en-GB/ˈbɜːnɪŋ ˈɡlɑːs/
en-US/ˈbɝnɪŋ ˈɡlæs/
large convex lens that can concentrate the sun's rays onto a small area, heating up the area and thus resulting in ignition of the exposed surface