If you're interested the Webb mirror, and the relationship between resolving ability, wavelength and mirror size I really recommend the deep dive Huygens optics did on the subject.

https://www.youtube.com/watch?v=gOpbXBppUEU

The guy makes his own lenses, and loves to explain what he does

Cool! Just don't make it curved, like the original Webb mirror - or, if you do, make sure to never ever let direct sunlight hit it...

For real though, focussing something like that is not easy. Unless you're measuring your angles really carefully you're probably not going to create a death ray.

There have been some attempts (including a couple by Mythbusters) to replicate the legendary heat ray of Archimedes, but it looks somewhat implausible with the tech of the time.

https://en.wikipedia.org/wiki/Archimedes%27_heat_ray

However Richard Hammond did have an toy experiment at Keck with a mirror with a diameter of about a metre and did have some success: https://www.youtube.com/watch?v=ix_tsqKzkrQ&t=3384

I once cooked an egg with the sunlight hitting the window/mirrors of the Cité Mondiale building in Bordeaux. It was a really hot day, it took about an hour, it was the best fried egg I ever had. https://www.beam.fr/phototheque/?context=location%7C83

This article is interesting in itself but I found a lot more by clicking the links in the left sidebar. The blog reminds me of the old web in the best possible ways, kudos to the author.