As someone who grew up consuming a lot of superhero-themed content that was anything but the actual source material, I always felt there was a little – maybe a lot – of knowledge I was missing out on. So a couple of years ago I dove into the Marvel Comics Universe (Some might say the original MCU … no, that’s only me?) with Fantastic Four #1, and proceeded in chronological order.

Before even finishing that first tale, I realized I had been handed a great tool to talk about science. There are loads of people who use comic books (or their spin-off products) for that purpose, of course. Like that time I thought I had invented the concept of exercise equipment with built-in television screens,1 this was by no means an original idea.

But those people usually write or make videos about the science behind or feasibility of the big topics, like how Mjolnir works, or how Iceman’s powers constantly violate the laws of thermodynamics, or how mass is never conserved, ever.

No one is out there using the third panel from the second story in the Nth2 issue featuring Spider-Man … that has absolutely nothing to do with Spider-Man … to talk about how light works:

Narration: "While inside, the Torch uses his infrared glowing power to dazzle his teenage friends!" Human Torch says "Here, Sally! I'll put an infrared spotlight on you while you dance!" Sally says "Mmm! Wait till actors union hears about you, Johnny!"

The Amazing Spider-Man #8

Usually I give these panels some context – what’s happening with regard to the plot in the comic. Here, you don’t need to know it, like, at all. The second story in this comic is just a dumb fight between Spidey and his archnemesis, sometimes known as Johnny Storm. They’re always going out of their way to fight because each thinks the other is a glory hound. Teenagers.

Those of you who remember learning about the electromagnetic spectrum might have a bit of a problem with Johnny Storm’s terrible science knowledge. The purpose, after all, of a spotlight is to make something more visible – whether it’s an escaping perp or a bit of cloud surrounding the approximate shape of a chiropter.3

First appearance of the Bat Signal, in Detective Comics #60, 1942

And infrared light isn’t visible to human eyeballs!

a chart of the electromagnetic spectrum, from radio waves to gamma rays, showing how small a range visible light takes up

Look at all that light we can’t see! Credit: NASA

Actually, that’s not exactly true. You see, Nature doesn’t care about how we puny little humans decide to categorize things. We can’t all agree on where red ends and infrared starts, but the wavelength you’ll see separating them is usually one of two values: 700 or 750 nm … sometimes there’s a 720. Light on the shorter end of that number is classified as “red,” light on the longer end is classified as ‘infra(whichisLatinforbelow)-red’.

But some studies using lasers (e.g. 1, 2) have demonstrated that, at least under special conditions, humans can perceive light with wavelengths up to 1,000-ish nm (otherwise known as 1 μm, otherwise known as 0.000001 m). That light is solidly in the subcategory abbreviated “NIR.”4

So, it depends on the exact wavelength of light the Human Torch is attempting to shine on Sally, but if it’s smack dab in the middle of the IR band, no one’s gonna see it unless they’re wearing special goggles that can translate the light into something their eyes can actually detect.

A false-color image showing the light a human body emits in the infrared

Yours truly as seen by an infrared camera; yes, this is basically a fake mirror selfie. Note that the colors are arbitrarily defined so that warmer areas are redder; in the real world, warmer objects actually glow bluer. Credit: Me

The other reason why Johnny needn’t shine an IR spotlight on Sally is that she’s already literally glowing in the infrared. So are all the other humans in that panel. So are you.

As you can see from NASA’s fancy infographic above, your typical human body radiates infrared light with a wavelength about 10 μm. You actually emit a range of wavelengths. The 10 μm value is the “peak wavelength”, or the wavelength of the highest percentage of your body’s photons.

Yes, there is a formula you can use to estimate that peak wavelength. It’s called Wein’s Displacement Law, depends only on a body’s temperature, and it looks a little (well, exactly) like this:

λpeak (in meters) = 2.898 × 10-3 ÷ T(in Kelvin)

If you plug in an average human skin surface temperature, like 307 K, you get a peak wavelength of 9.43 μm, which is rounded to 10 for back-of-the-envelope calculations. However, there’s an entirely separate (and far mathier) equation to show the range of wavelengths the body is emitting at that same temperature. The resulting curve looks like this:

A blackbody curve approximating a human body (based off surface skin temperature of 307 Kelvin)

All you need to know about the vertical axis is that it represents a count of the photons with each wavelength. The higher the curve at a particular wavelength, the more photons there are with that wavelength. That super skinny rainbow at the far left indicates where the visible wavelengths are: 0.4 nm to 0.75 nm.

It looks like all the light being emitted is in the infrared, but if we zoom in on the visible part of the spectrum, we see an isty bitsy, teeny weeny number of photons of visible wavelengths.

Blackbody curve approximating a human body (based off skin temperature of 307 K), only at visible wavelengths

Yes, the vertical axis numbers are roughly a billion billion times smaller than those in the previous graph. I *did* say itsy bitsy teeny weeny.

[It’s important to note that this curve won’t match a real measurement taken from a human body. It’s based on an equation that only depends on one variable (a single temperature value) and assumes that the radiating body in question only emits light – it reflects absolutely none of the other light that hits it. Humans clearly do not do that; if they did, we wouldn’t be able to see them.]

This is the visible light that a human body emits just by having a temperature higher than absolute zero.5 Humans also emit visible light due to chemical reactions that take place on the cellular level – our own form of bioluminescence – but it’s a thousand times weaker than our eyes can detect.

All that being said, there is a way that Johnny could actually make Sally glow more in the visible range of the electromagnetic spectrum.

He could set her on fire.

If you’re looking for way more of this kind of analysis (like, over two years worth of weekly posts “more”), you can head on over to my blog BadBackgroundScience. You’ll find all the classic ’60s Marvel heroes covered – at least those that have made appearances before 1964 – and a running series in which I attempt to discover what Doctor Doom’s doctorate is in … or at least what it is not in.

1. I still like to think I had that idea at a young enough age that none of them actually existed, yet. We all have our delusions…
2. Spider-Man’s first appearance was not in an ‘actual’ Spider-Man comic, but the anthology Amazing Fantasy #15. Also, he’s popped up in some of the other Marvel comics for some crossover stuff. It usually involves fighting the Human Torch.
3. Fun bonus science fact: Bats aren’t “flying rodents”. Rodents belong to the taxonomic order Rodentia, whereas Chiroptera is its own separate order made up entirely of bats. Rodents comprise about 40% of all mammal species; bats comprise about 20%. Second fun bonus science fact: Some bat species have lousy vision, but none are outright blind. Bonus maybe not as fun etymology fact: “Chiroptera” derives from the Ancient Greek for “hand” (χείρ) + “wing” (πτερόν).
4. That’s Near-Infrared; it means it’s the section of IR that’s nearest to visible, not that it’s near the infrared band. It runs from wherever people define red down to ~2.5μm. There’s also a MIR (Mid-IR, ~2.5 to ~25μm) and FIR (Far-IR, ~25 to ~1000μm, where it changes from IR to microwave). I’ve scienced in all of them. Go me.
5. Even the tiniest of dust particles and random gas atoms floating in the (otherwise) empty void of space are sitting at a non-zero temperature, and therefore radiate photons. But those are super invisible to human eyes. The least massive stars – aka red dwarfs – have surface temperatures between 2-4,000 K and a peak wavelength in the NIR, but emit enough red photons that telescopes can see them (if they’re close enough to Earth). If you have perfect vision and have perfect viewing conditions, you might be able to see one red dwarf in the night sky: Lacaille 8760. Our Sun’s surface has an average temperature of ~5800 K, and a peak wavelength of 500 nm, which corresponds to a bluish green. Sunlight is how humans defined white, so the Sun is white. Hotter stars have shorter peak wavelengths and look bluer.