Video: The internet is a wonderful place. Case in point: there's an entire YouTube channel called Let's Melt This, devoted to videos of, well, stuff melting. In the latest instalment, the team placed two pennies under a Fresnel lens to see how long it would take for them to melt under the concentrated heat of the sun. Unlike a standard magnifying glass, which is thick in the centre and tapers off tower the edges, a Fresnel lens is thin — little more than a flat plastic sheet with ridges on one side. It was invented in the 19th century by a French physicist named Augustin Jean Fresnel and originally used in lighthouses to concentrate the beam so the light could be seen from as much as 32km away. Today they're found in old rear-projection TVs and SLR camera screens. And they're terrific as solar concentrators, whether you want to do some off-the-grid cooking, or just melt stuff for entertainment.
It takes less than a minute for sunlight concentrated through a Fresnel lens to melt the top penny. Watch, and take a moment to reflect on the awesome power of the sun. And if you're feeling particularly ambitious, maybe try your hand at building your own Fresnel lens "death ray".