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 light and about 5
minutes for the fire to ignite. Basic rules, p. 51)
None of the rules indicate heat as a characteristic of various light spells, although some characteristics of spells that are "sunlight" also cause radiant damage. The light cantrip is not described as sunlight, nor "as bright as sunlight."
Sunburst / Evocation 8th level / Bright sunlight in a
60 foot radius centered on a point you choose within the range.
And that spell doesn't last the required 5 minutes.
It is not clear whether the "radiant" damage includes or not "heat" or if it is some kind of magical sacred radiation. Not all light sources can do what the sun does.
- For example, my LED flashlight illuminates our kitchen quite well
when I hang it from the beams, but I can't concentrate that light
in something that will cause a fire. Yes, I tried a challenge from my son.
Is this a great idea to light a fire? Yes.
Under Rules as fun, I could rule as DM that with a little time and effort, a clergyman could (with a suitable magnifying glass or with a similar element like a lens of a spyglass (Expensive!)) Concentrate enough heat of light on something that ignites a fire. But a tinderbox is much cheaper. Do you want to disassemble that spyglass and may not be able to use it?
1,000 gp spyglass
100 gp magnifier
Tinder 5 sp.
(Basic rules, p. 51)
There are a couple of other cantrips that can light a fire as part of their basic function: druidcraft, produce flameor that old standby prestidigitation
• Instantly light or extinguish a candle, a torch or a small
bonfire. (Basic rules, p. 102)