I was wondering about tunnels with mirrors, but it doesn't seem enough. And I want it to be physical source, not magical. The world doesn't rely on magic too much. Magma's and bioluminescence's light isn't strong and beneficial enough either.

There are species with no darkvision, so I need light.

Magma or giving creatures native to the area darkvision.

There's a lot of lore in both fiction and tinfoil hat conspiracy and cryptozoological space that explains hollow earth workings, but sometimes there be dragons in those spaces so I wouldn't necessarily recommend research unless you're comfortable with some dark rabbit holes.

Are you trying to tunnel and mirror sunlight?  I'd reverse that.  There can be a lot of heat in the layers within a planet.  Molten cores, and lava flows etc.  Where there's heat there's often light. So that's your source.  Parts of the world radiate heat upwards and the composition of the underside of your world's crust or the aspects of the atmosphere make it clear as day so to speak.

What I'd see as more of a problem is light being largely a static condition perhaps affected by interior atmosphere conditions.  That is your hollow world would have dark zones and light zones, but no day or night time.

Will any of this satisfy a geophysicist, probably not, but few world ecologies are really so realistically thought out.  Just a rough idea if you want to run with it.

2e had a hollow earth setting, Mystara.

Magma and bioluminescence are probably the most common tropes of underground light.

Quote from MidnightPlat >>

There's a lot of lore in both fiction and tinfoil hat conspiracy and cryptozoological space that explains hollow earth workings, but sometimes there be dragons in those spaces so I wouldn't necessarily recommend research unless you're comfortable with some dark rabbit holes.

Are you trying to tunnel and mirror sunlight?  I'd reverse that.  There can be a lot of heat in the layers within a planet.  Molten cores, and lava flows etc.  Where there's heat there's often light. So that's your source.  Parts of the world radiate heat upwards and the composition of the underside of your world's crust or the aspects of the atmosphere make it clear as day so to speak.

What I'd see as more of a problem is light being largely a static condition perhaps affected by interior atmosphere conditions.  That is your hollow world would have dark zones and light zones, but no day or night time.

Will any of this satisfy a geophysicist, probably not, but few world ecologies are really so realistically thought out.  Just a rough idea if you want to run with it.

 Light from magma will not be beneficial, due to it is not nutritious as sunlight (vitamin D + plants), and magma's light isn't strong enough to light.

You may also want to consider bioluminescence. You can explain that plants and fungi that gives off more light are also more likely to have their seeds and spores propagate since animals can see them better.

Quote from Emmber >>

Magma and bioluminescence are probably the most common tropes of underground light.

 But it's not strong enough to light a whole hollow world

Quote from XXXGammaRay >>

You may also want to consider bioluminescence. You can explain that plants and fungi that gives off more light are also more likely to have their seeds and spores propagate since animals can see them better.

 Not strong enough to light up a whole hollow world

Look at the underdark in most worlds. 

Quote from Skfma >>

Quote from XXXGammaRay >>

You may also want to consider bioluminescence. You can explain that plants and fungi that gives off more light are also more likely to have their seeds and spores propagate since animals can see them better.

 Not strong enough to light up a whole hollow world

Evolution is an pretty amazing thing, bestowing life with stuff like flight and intelligence. I do not think it would break any physical laws to have bioluminescence to be bright enough to illuminate a hollow world, provided that there are enough organisms with that feature. I think most or all bioluminescence is achieved via chemical processes, but I do not think it is physically impossible to achieve bioluminescence powered by radioactive decaying elements either.

If you really do not like the idea of shining plants and fungi, illumination could also may be powered by radioactive elements in ores and rocks, although I do not know any rocks that glows brightly in the visible spectrum in the real world.

I think you are going to have to rely on unnatural light, which works since the world it self would require some unnatural methods to prevent it collapsing in on itself, provide oxygen, maintain survivable pressure and "normal" gravity.

I get it, but as I tried to imply in my quip about satisfying a geophysicist, you're going to have to bend the laws of physics and biology if you want a hollow earth that a flora and fauna like a traditional surface world or go the underdark route where there isn't so much vegetation as a mycology ecology and relatively acceptable low light and no light conditions.  And if Vitamin D arguments and light levels are going to be the test to fail your game world, good luck with Dragon metabolism or the equivalent.  You need a source of sunlight, you need to carve a way for that light system into the interior, or have a geothermal affect that provides the light equivalent of a star's thermonuclear effect "interior" to your planet ... which would likely "realistically" create an inverted Dark Sun world where skin cancer is the greatest predator.  

Maybe just stick with a magic crystal core that pulsates diurnally with a dawn and dusk cycle. 

You can't have a hollow world without any magic. You're going to have to bend some laws of reality.

Well technically, you're making the reality, and like literally saying "Let there be light' in this case :)

I do like the idea of a world that's so bright in its interior, Drow wear sunglasses, Duergar wear large brimmed trucker hats, and everyone needs to invest in sunblock concocted by Illithids.  "I touched the sun" isn't so much an aspiration invoking legend but a common domestic hazard during morning stretches leading to many sick calls from work in the society.

Living crystals, that shine with a bright yellow light for 12 hours, then fade to a pale blue light for another 12 hours.  They are simply "there" as are stalagmites in places or any other natural stone formation.  They consume carbon and emit oxygen in cycles to explain the color shifting.  As natural in your underworld as ferns are in our world.

World is actually a dyson sphere, with a small sun at the centre, only people on the outer surface don't know?

Permanent huge (10,000 ft wide ) Portal at the center of the planet that leads directly to the Elemental Plane of Fire.

Nothing gets near it except for fire, lava and elemetnal earth creatures.  

Large walls (not columns, walls  of molten lava magically hold the Portal in place.  These lava walls actually seperate the core into about 8 different central caverns that each can see the gate. Some of those 8 are full of lava.  Others are air filled caverns that people live in.  They are constantly in bright light.

Welcome to the 'underbright', the hidden world beneath the underdark.

Welcome to the 'underbright', the hidden world beneath the underdark.

That is awesome.

To the OP - you might want to look up the Pellucidar stories by Edgar Rice Borroughs. That is his take on a world inside of a "hollow earth." It's pretty cool stuff -- as is just about everything ERB ever wrote.

Assuming there is still a "top world" where light reaches but is simply unreachable for the people in the underworld...  You could have trees which absord the sunlight from the inside and use it to make their roots glow with a faint light... To make it interesting maybe they arent regular trees.. maybe it's a type of predatory plant which absorbs sunlight, makes their deep loot system glow with light to lure in creatures underground... once the creatures get too close to the roots, maybe the roots and can stun and kill it's victims and absorb their bodies into the roots.