I'm building a magic system where magic isn't just controlled by human brains, but is actively produced there. As a byproduct, when casting a spell, the brain generates a large quantity of visible light (but not infrared light or heat). I don't know much anatomy, but I would imagine that, since human skin, eyes, and muscle tissue are all translucent to varying extents, it might be possible for people looking at the mage to see this light. It would mostly emanate from their eyes and nose, where their skull isn't in the way.

My question is: is this really how it would look if enough light was produced by the brain? And if so, what would be the health effects (i.e. could the person still see? Would brain damage be inevitable with repeated exposure?) Or would an intensity strong enough to see from outside the skull be guaranteed to fry the brain or something?

Note that while the light in this question is magical in origin, I don't want to have any arbitrary additional magic that protects the brain or rest of the body. Just accept that the person's brain is glowing (don't worry about where the energy is coming from), and use regular physics and biology from there.

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    $\begingroup$ Most of that light will be absorbed by the blood, bones, muscles and skin and promptly coverted into, guess what, heat. As for the strange belief that there is no skull bone behind the eyes and the nose, the ethmoid and sphenoid bones (which sit respectively behind the nose and the eyes) would like to have a word. (Hint: the neurocranium is mostly a closed box, with one large opening (where it articulates with the spine) and several small openings.) $\endgroup$
    – AlexP
    Commented Aug 25, 2019 at 17:32
  • $\begingroup$ Huh. See, those bones are what I meant when I said I don't know much anatomy. Those are news to me :P $\endgroup$ Commented Aug 25, 2019 at 19:12
  • $\begingroup$ Most of the named bones are very thin, though. Transillumination won't be a problem through the eyes, nose and even mouth. $\endgroup$
    – elemtilas
    Commented Aug 28, 2019 at 15:30

4 Answers 4


Reality Check = OK!

Transillumination of body tissues is a thing. Thin tissues work best: if you close your eyes while facing a bright light, you can discern the shadow of you hand as you pass it back and forth before your face.

Certain (minimally invasive) surgical procedures rely on the phenomenon in order to determine if an instrument is placed properly. You are correct that eye sockets and nose will shine brightest: a reddish~yellowish glow. Depending on how bright the brain's dwimmerlight is, you may also find a glow from the forehead and perhaps mouth as well.

enter image description here

As for your add-on questions, the light shouldn't affect the brain itself, as there is not heat generated (beyond, I suppose, the heat generated by ordinary thaumo-metabolic processes). Since this is visible but non-IR light, your mages will be able to see it. The vision receptor cells in the retina are physically stimulated by light photons, so there's no real getting around that. It only becomes a question of how bright you want the light to be, which if powerful enough could cause temporary or even permanent damage. Just like any bright light source.

Even though the dwimmerlight itself doesn't produce heat, it's been pointed out that as it's absorbed (in tissues), some will indeed be converted to heat. I think it would be wise for a busy wizard to have a bit of a lie down in a cool place every hour or so of heavy spellcasting! His circulatory system should be able to handle a low amount of heat transfer, but too much will be injurious.

That's simply the price of magic in your world!

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    $\begingroup$ Since any absorbed light is converted into heat, too bright light might damage also other things than the eyes. Which would give a natural limit to the strength of your magic, assuming you want to survive it. $\endgroup$
    – celtschk
    Commented Aug 25, 2019 at 21:26
  • $\begingroup$ @celtschk --- I make no guarantees mages will survive their magic for any length of time! (Amended.) $\endgroup$
    – elemtilas
    Commented Aug 25, 2019 at 21:29
  • $\begingroup$ The question is about light generated by the brain, which is encased in the skull, which would block/absorb most of the light. See AlexP's comment on the question. $\endgroup$
    – Alex B
    Commented Aug 28, 2019 at 13:00
  • $\begingroup$ @Haihami --- Yes. As you can see in the picture, that is light passing through bone as well as soft tissue. $\endgroup$
    – elemtilas
    Commented Aug 28, 2019 at 15:22

Human tissue, other than parts of the eye, is not transparent.

Assuming for arguments sake, the light is generated at the center of the brain, to keeps things simple. As the photons travel outward, some percentage will be absorbed by neurons. When this happens the temperature of the neuron increases by a small amount. If a neuron absorbs enough photons, then its temperature will increase enough to thermally activate. The neuron will fire and transmit its signal to other neurons is it connected to.

This is all happening randomly. If a small number of neurons fire the brain function can be compromised, temporarily. This could be anything from a grand-mal seizure to remembering what you had for breakfast 42 years ago.

Since the brain is mostly fat and protein, if neurons absorb enough heat, they’ll coagulate and that circuit of the brain will be compromised. If that is your memory of third grade, then its gone. If its a motor neuron control a muscle used for breathing, the you’ll experience a reduced ability to inhale and exhale. Not fatal, if its only one motor neuron, but after some number are destroyed you are dead — this is what dying from ALS or Lou Gehrig’s disease is like.

I think light can leak out through penetrations in the skull — so the eyes might light up (and this would interfere with seeing to some, the ears, and maybe the throat through the penetration at the bottom of the skull for the spinal cord.

Generally, our skulls are pretty much opaque and our thickest parts of our skull are facing forward and the back of our skulls.

You might look at this Interactive 3D exploded view of human skull to see where you think the skull is thin enough for light to leak out

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    $\begingroup$ I pictured most of the light being produced at the surface of the brain, though now that you mention it, I can't think of a good reason why that should be the case. The magic is supposed to be caused by electrical patterns, which should be most concentrated in the interior of the brain. Anyway, I didn't specify either way, so this is a solid answer. Cool skull animation, too. $\endgroup$ Commented Aug 27, 2019 at 8:39

Congratulations, it's a wizard.

Yes, the skull is mostly a closed bone box -- in adults.

Babies have 'soft spots' -- fontanelles -- in their skulls, where the plates haven't quite closed up yet.

If you've got glowing human brains, early infancy would be a good time to evaluate their luminary potential.


So some people shone some incandescent lights into the heads of dead people to see what gave. Turns out that the translucency of skull bones among people spans six orders of magnitude. In other words, one subject was a million times more translucent in the head than another subject. Some people simply have very thick skulls.

But truth be told, there you have it, some skulls might shine like a reflector whereas others might go unnoticed - from the top. You still have a hole in the bones between the brain and the eyes, where the optic nerve passes through.

We are all like that inside Source: https://www.kenhub.com/en/library/anatomy/the-optic-nerve

Practically every tissue in the body is orders of magnitude more translucent than bones. Any mage with even a slightly perceivable shine on their heads would shine a blinding light from their eye sockets. And you know what is in there? The retina.

Casting any spell will blind the mage and possibly blind anyone looking at their eyes. And this is more probably permanent for the mage.

Who knows? This may have been the actual reason behind Laser Pony's accident.

Feel the gaze of the Laser Pony!

  • $\begingroup$ Yeah, it definitely seems like if the light is bright enough to be seen through the skull, it'll blind the mage and shine out much more strongly through their eyes. But that implies that there's a much lower brightness that can only be seen as a dim glow through the eyes, right? I mean, I'm the worldbuilder. If I want to give my mages glowy-eyes for rule-of-cool reasons (that are justified by the way magic works in my story), then I could just make the brightness lower. $\endgroup$ Commented Aug 28, 2019 at 11:17
  • $\begingroup$ @GiladM yes and yes. $\endgroup$ Commented Aug 28, 2019 at 11:40

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