Often on artwork you see heros and villains with glowing eyes, a sign of a sort of supernatural or technological power they possess.
Why do glowing eyes convey this? I assume it’s a cultural norm, but perhaps there’s a deeper element to it.
Window to the soul, and all that…
I’d guess that there’s some element of it being reminiscent of predatory animals with good night vision (some of which appear to have glowing eyes thanks to the way their retinas reflect it).
There’s also the convention of glowyness being shorthand for “powerful”.
Glowing things tend to be powerful or dangerous, like fire, lightning, lava, the sun, etc.
I wonder if it started with Superman’s laser vision. It seems pretty to be a pretty natural progression from
“Superman has a power that makes his eyes glow”
to
“Hey, eyes glowing look pretty cool, and make it look like shit is about to go down because he’s about to shoot it.”
to
“Generic signifier that the person is powered up and shit is about to go down.”
In general, the eyes are the focal point of anyone’s face, and in fantasy and science fiction, the eyes are the one area where you get the most bang for your buck in alteration of appearance. Sure you could make someone’s ears or nostrils glow, and I sure as fuck would piss myself in real life, but on film or in comics it kind of comes across as silly. There’s something mystically internal about the eyes that no other body part holds, too. They feel like they’re connected to your brain, if not your very soul.
That said, the glowing trope has been around for ages. If you were presented two rolls of toilet paper, one glowing the other not, at the least I’d have some questions about the glowing roll and use the non-glowing roll, at the most I’d run. So it goes for eyes as well.
Yeah! I like the way Peter Jackson imagined Smaug’s chest and rib-cage when he’s about to incinerate someone! That was one bad-ass dragon!
I don’t know the oldest description of glowing eyes, but these are a start:
Revelation 19:12
His eyes are like blazing fire, and on his head are many crowns. He has a name written on him that no one knows but he himself.
Revelation 1:14
The hair on his head was white like wool, as white as snow, and his eyes were like blazing fire.
Virgil describing Charon:
There Chairon stands, who rules the dreary coast -
A sordid god: down from his hairy chin
A length of beard descends, uncombed, unclean;
His eyes, like hollow furnaces on fire;
A girdle, foul with grease, binds his obscene attire
Shittiest superhero origin story ever.
Captain Charmin and his sidekick Two-Ply!
Think about it: have you ever personally seen anyone possessed by supernatural powers who didn’t have glowing eyes?
And their arch-nemesis, Lord Uranus 
Glowering eyes are pretty powerful also.
Word has it he only has one eye, and it glows something fierce!
Also from the Christmas hymn, “The Angel Gabriel from Heaven Came”:
“The angel Gabriel from heaven came,
With wings as drifted snow, with eyes as flame:
‘All hail to thee, O lowly maiden Mary,
Most highly favored lady.’ Gloria!..”