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The light spell provides a 20-foot radius of bright light and 20 additional feet of dim light.

But a hooded lantern provides 30 feet of bright and 30 more feet of dim light (and a Bullseye Lantern provides a 60-foot cone of bright light and dim light for an additional 60 feet).

Any reason you couldn't cast light on a small object and pop it into one of these items to get the same performance?

I tend to think "yes" because this is as simple as the difference between a naked bulb, and a bulb with a specific arrangement of reflectors around it.

Plus the light spell casts light in all directions, the hooded lantern reflects light in a plane and the bullseye lantern in a narrow cone. It's not like this isn't basically covered by the laws of physics.

I can even imagine specially built lanterns with an arrangement to make it easier to cast light on a copper piece and then drop the copper into a specially built slot.

But I'm fine with the light spell being cast on "the lantern's wick" - And even if you think light lights up the whole object (see similar questions) there's no reason the copper piece trick wouldn't work (or pulling the wick, casting light on that, etc.)

Am I missing something? Or does this open up the D&D world for a special type of lantern designed for the light spell?

I think this question boils down to - Can one use existing D&D world 'technology' in combination with magic?

The existing technology is "Reflectors work well to channel light sources" - So what's wrong with combining common knowledge with a magical light source?

On the other end of the spectrum, if the question was "Can I build an electromagnet and power it with a lightning bolt?" the answer would be "Hells no! Nobody in the D&D world knows what an electromagnet is!"

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    \$\begingroup\$ ...What is your actual question? \$\endgroup\$
    – V2Blast
    Jul 19, 2018 at 7:17
  • \$\begingroup\$ Also, another very related question asked by you: Can I use a Light spell instead of oil in a Lantern of Revealing? (As written, this question seems sort of like a duplicate, except this question seems more like a discussion prompt - which would be inappropriate for a Q&A site like RPG.SE.) \$\endgroup\$
    – V2Blast
    Jul 19, 2018 at 7:17
  • \$\begingroup\$ The original question was in two parts. Then someone edited it down to one and suggested I ask this separately. So here I am, asking again. \$\endgroup\$
    – Σ of eDπ
    Jul 19, 2018 at 23:14
  • \$\begingroup\$ "Can one use existing D&D world 'technology' in combination with magic?" - this question is still too broad. What I think you're actually trying to ask is, "If I cast Light on the wick of a lantern (or on another object placed inside the lantern), would it provide the same lighting as the lantern normally would?" Is that correct? \$\endgroup\$
    – V2Blast
    Jul 19, 2018 at 23:42
  • \$\begingroup\$ Way too specific. Plus I'm happy with the answer I got before you put this on hold as they offered sensible, general guidance around (what I thought was) a sensible, general question. \$\endgroup\$
    – Σ of eDπ
    Jul 20, 2018 at 0:05

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Any reason you couldn't cast light on a small object and pop it into one of these items to get the same performance?

I tend to think "yes" because this is as simple as the difference between a naked bulb, and a bulb with a specific arrangement of reflectors around it.

The interaction between magical effects and logic based on real-world physics is a murky area that in general could get you into trouble.

If:

  • It makes sense to you and others playing with you

  • It does not cause odd rulings or balance issues elsewhere in the game

  • It does not waste time on arguments around the table

then all is good.

In this case, the PC carrying the lantern is paying the full cost of both obtaining the object and needing a free hand to use it effectively. Notably, in combat, a lantern is not as convenient to the bearer as a e.g. a Light spell cast on a helm or shield. So the balance is mainly using a spell versus cost and weight of fuel. In a game where fuel use and/or weight is not strictly tracked (which is a common simplification at many tables), then the difference is almost nothing.

However, there are good arguments that is not RAW use of the spell (because the spell does what it says it does), so if you have people in your group that like to play strictly by the rules, it is probably not worth pushing the idea too hard.

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    \$\begingroup\$ Magic isn't science. I've always found that doing anything other than doing exactly what the spell says is a very, very slippery slope. Don't do science, kids, its how you lose friends. \$\endgroup\$
    – JackChance
    Jul 19, 2018 at 16:00
  • \$\begingroup\$ Good point - I added a specific question and counter-example for clarity. \$\endgroup\$
    – Σ of eDπ
    Jul 19, 2018 at 23:22
  • \$\begingroup\$ Thanks for the guidelines. This will be good for both allowing and disallowing items. Ex. If someone wants a light spell powered, mag-light equivalent mounted on their crossbow the answer will be no. Because while a lantern is an existing tech in 5e, the flashlight (let alone the tactical flashlight mount) is not. \$\endgroup\$
    – Σ of eDπ
    Jul 23, 2018 at 22:09
  • \$\begingroup\$ @SumofeDpi while a flashlight (hand-held directional light tube) is not medieval tech, the bullseye lantern and the lighthouse (one of which appears in the basic rules, the other in official adventures) use a similar principle of selectively placed shades, reflectors, or lenses to guide and focus light in particular directions. So I would tend to allow this to replace an actual flame in existing lantern designs, but not to invent a tubular flashlight that you clip onto a weapon or hold in your hand. And hanging a bullseye lantern off a crossbow could throw off your aim. \$\endgroup\$ Mar 25, 2021 at 16:48

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