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Theoretically, you can light a piece of paper with a heat-emitting light source and a powerful enough magnifying glass as this topic seems to explain.

The cleric spell Light allows an object to shed "bright light in a 20-foot radius". My interpretation is that the object itself become a light source and possibly warmer (like a lightbulb) but I didn't find any official information regarding if the light the object emits produces heat or not.

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    \$\begingroup\$ Relevant xkcd \$\endgroup\$ – Punintended Nov 18 '19 at 21:17
  • \$\begingroup\$ Light bulbs get warm because they're nowhere close to 100% efficient; much of their electrical power turns into heat directly, never becoming light. (Even an LED lightbulb is only ~20% efficient at converting incoming AC power into visible light power that actually makes it out of the device.) \$\endgroup\$ – Peter Cordes Nov 19 '19 at 4:10
  • \$\begingroup\$ Photons of visible light do carry some energy, so holding your hand around something you cast "Light" on should in theory give some warmth barely-measurable warmth, but you shouldn't expect the object itself to have warmed up at all: no reason to assume the Light spell isn't perfectly efficient or that any of the light energy is absorbed by the object unless you reflect the light back at it. And it really doesn't take much light power to be perceived as "bright" by humans, if it's all emitted in the visible range, not also infrared (like blackbody radiation from a tungsten filament.) \$\endgroup\$ – Peter Cordes Nov 19 '19 at 4:12
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Not by RAW: "bright light" =/= "sunlight" =/= "The Sun"

Magnifying Glass. Lighting a fire with a magnifying glass requires light as bright as sunlight to focus, tinder to ignite, and about 5 minutes for the fire to ignite. Basic Rules, p. 51)

None of the rules indicate heat as a characteristic of the various light spells, although some "are sunlight" and can do radiant damage. The light cantrip is not described as being sunlight, nor "as bright as sunlight." Light emitting spells that might suffice:

  • Sunburst

    8th-level evocation / Duration: Instantaneous
    Brilliant sunlight flashes in a 60-foot radius centered on a point you choose within range.

    That spell doesn't last 5 minutes.

  • Daylight.

    3rd level / Duration: 1 hour
    A 60 foot radius sphere of light spreads out from a point you choose within range. The sphere is bright light and sheds dim light for an additional 60 feet.

    It lasts long enough, but isn't sunlight.

  • Light

    Cantrip / Duration: 1 hour
    You touch one object that is no larger than 10 feet in any dimension. Until the spell ends, the object sheds bright light in a 20-foot radius and dim light for an additional 20 feet.

    Lasts long enough, but it isn't sunlight.

  • Dawn

    5th level / Duration: Concentration, up to 1 minute
    The light of dawn shines down on a location you specify within range. Until the spell ends, a 30-foot-radius, 40-foot-high cylinder of bright light glimmers there. This light is sunlight.

    Yes sunlight, not a 5 minute duration.

Note: Whether or not radiant damage includes "heat" or is some other kind of magical / holy radiation is unclear. Not all light sources can do what the sun does.

  • For example, my LED flashlight illuminates our kitchen pretty well when I hang it from the rafters, but I can't concentrate that light into something that will start a fire. Yes, I tried on a challenge from my son.

Is this a cool idea for lighting a fire? Yes.

Under Rules as Fun, you could rule as a DM that with a bit of time and effort, a cleric could (with a suitable magnifying glass, or with a similar item like a lens from a spyglass (Expensive!)) concentrate enough heat from the light into something that starts a fire. But a tinderbox is a lot cheaper. Do you want to dismantle that spyglass, and perhaps not be able to use it?

Spyglass...1,000 gp
Magnifying glass...100 gp
Tinderbox...5 sp.
(Basic Rules, p. 50)

There are other cantrips that can light a fire as part of their basic function: druidcraft, produce flame, or that old standby prestidigitation

• You instantaneously light or snuff out a candle, a torch, or a small campfire. (Basic Rules, p. 102)

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  • \$\begingroup\$ Thanks for all the informations ! The tinderbox sure is the simpler way for a Cleric to light a fire \$\endgroup\$ – Oddrigue Nov 18 '19 at 14:18
  • \$\begingroup\$ Note that the rule only specifies “as bright as sunlight”, it doesn’t require sunlight. Now sunlight is as bright as sunlight, and it is unreasonable to expect most artificial light sources to be as bright as sunlight (it is quite well known that direct exposure to sunlight can blind, which most light sources aren’t intended to do), but that a light source isn’t specifically called out to be daylight shouldn’t disqualify it from being a source of bright light for a magnifiying glass. \$\endgroup\$ – Cubic Nov 19 '19 at 14:11
  • \$\begingroup\$ @Cubic The problem with that comment is that on an overcast day you can have daylight, but not sunlight. Daylight and sunlight are not synonyms. The game terms are "bright light" and "sunlight" I can have a flashlight that is bright that isn't as bright as sunlight. In fact, I do. \$\endgroup\$ – KorvinStarmast Nov 19 '19 at 14:12

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