I want to burn my foes like ants!

Question

Inspired by this image…

…I'd like my PC to create a similar mundane contraption that turns sunlight into a weapon. Are there mechanics somewhere in 3.5 that make such a device possible?

Answer 1

Well, a magnifying glass is a statted item in D&D 3.5e, so in theory this could be possible. It’s said to cost 100 gp, but no DC is given to craft one. The Craft skill description does include this, though:

Item	Craft Skill	Craft DC
Very simple item (wooden spoon)	Varies	5
Typical item (iron pot)	Varies	10
High-quality item (bell)	Varies	15
Complex or superior item (lock)	Varies	20

So that suggests a DC of 20 to craft a magnifying glass: while not complex, magnifying glasses require extremely fine and near-perfect work. In order to focus like that, you need a very consistent curvature, and there needs to be zero blemishes or scratches on the surface. It is vastly harder to produce than a bell; in reality, it’s probably quite a lot harder than your typical lock.

The associated Craft skill would probably be Craft (lensgrinding). Lenses are not blown, so glassblowing wouldn’t apply.

The other issue is that producing the small lenses used by spectacles, spyglasses, and hand-held magnifying glasses are far, far easier to produce than larger lenses, as seen in your image. Keeping everything perfect as you get larger is just exponentially more difficult. Producing lenses on that scale is extremely difficult for us today. It simply wouldn’t be possible at the technological level shown in most D&D settings (even the relatively-advanced Eberron), at least not without magical assistance. A fabricate spell could maybe do it, though, and after you finished making it the item itself would be non-magical, so maybe that works for you.

Even with fabricate, though, you have to make a Craft skill check. What DC? The DC you would need to do it normally. Which, in this case, you can’t. Arguably, that may mean fabricate can’t do it. But fabricate is kind of vague about this kind of thing in general: there are loads of things you can’t make without the appropriate tools (no one is going to karate chop a sculpture out of a hunk of marble, after all). The reason you can’t make really big lenses in D&D is because the tools for doing so don’t exist. But if fabricate obviates the need for tools... I don’t know. We are deeply into “the DM makes something up” territory. If it were me, I’d probably just say “No,” if for no other reason than to avoid having to make something up here. Anyway, even if fabricate means you basically count as having the tools (and associated bonuses?), the DC for this should be stratospheric.

The final problem is, well, there are no rules for how much damage this should do. The magnifying glass we’re told about doesn’t offer any stats related to focusing light to burn ants, so there isn’t even anything we can extrapolate from. Again, your DM would just have to make something up there. Some amount of fire damage in a small area, presumably, but it’s not at all clear how much that should be or how you should use it. It being in orbit implies you need to somehow communicate with it (which will require magic) and wait for it to get into position, but the details there will matter a whole lot.

But we can say that this would make for a pretty awful weapon, at least relative to its size, cost, and complexity. To get enough power out of it to be effective against things your size, rather than ant-size, you need a lot more sunlight, which means a much larger lens but also means a much greater distance from your target. You would probably need that orbital satellite holding the thing—and any communication with something in orbit from the surface is going to require magic in the world of D&D. They don’t any understanding of radio communications, and there certainly aren’t any rules for you inventing it. If you can manage to do all of that, you can manage much more powerful forms of attack than this.

Answer 2

Considering the insane resources and the rolls required to craft such an item, in addition to other reasons brought up by KRyan, if you wanted to do this, I would say your best bet is actually to attain Cleric Level 9 or Druid Level 7 so you can prepare and cast the spell "Flame Strike".

Flame strike
Evocation [Fire]
Level: Clr 5, Drd 4, Sun 5, War 5
Components: V, S, DF
Casting Time 1 standard action
Range: Medium (100 ft. + 10 ft./level)
Area: Cylinder (10-ft. radius, 40 ft. high
Duration: Instantaneous
Saving Throw: Reflex half
Spell Resistance: Yes
A flame strike produces a vertical column of divine fire roaring downward. The spell deals 1d6 points of damage per caster level (maximum 15d6). Half the damage is fire damage, but the other half results directly from divine power and therefore not subject to being reduced by resistance to fire-based attacks, such as that granted by protection from energy (fire), fire shield (chill shield), and similar magic.

I believe this spell produces a similar effect to what you're looking for though, sadly, it doesn't produce a concentrated beam lasting for more than maybe 1 or 2 seconds.