aarg:
Your "death ray" loses intensity in proportion to the square of the distance. Google "f stop".
Just how does sunlight reflecting off a flat mirror spread?
The sun is 98000000 miles away (about 150 million meters) and the rays are so close to parallel they might as well be. The flat mirror reflection is going to be as parallel as the mirror is flat.
How else could I use a palm sized piece of polished steel to signal people an SOS from miles away?
If the mirrors are in a ring that can be aimed at the sun then they could all tilt inward the same to meet at a desired, changeable focus.
There's a solar power plant in Arizona that uses trough reflectors (need one tilt adjustment for sun path inclination) to heat oil in pipes to heat a big reservoir and run full power generation until about 11PM -- so much for solar only works when the sun is up!
Cooking with solar... there's so many youtube videos showing variants on an insulated box with black insides and a window on front it's not funny. From India one guy used pizza boxes with plastic wrap to cook food for a school without burning any fuel. In Canada one video shows solar cooking a large roast with 10 inches of snow all around, that took hours btw.
I wouldn't mind using a trough heater to heat air in a steel stovepipe. A PC fan could push air in the cool end and an oven at the other end circulate that around the food. Something would have to let cool air in to regulate temperature.
If 1000000 people didn't use gas or electric to cook just 1 day a week, that would save noticeable resources.