Germanium. It might sound like just another periodic table entry (number 32, to be exact), but in the world of infrared light, it’s anything but ordinary. A recent video by [The Action Lab] dives into the fascinating property of germanium being transparent to infrared light. This might sound like sci-fi jargon, but it’s a real phenomenon that can be easily demonstrated with nothing more than a flashlight and a germanium coin. If you want to see how that looks, watch the video on how it’s done.
The fun doesn’t stop at germanium. In experiments, thin layers of gold—yes, the real deal—allowed visible light to shine through, provided the metal was reduced to a thickness of 100 nanometers (or: gold leaf). These hacks reveal something incredible: light interacts with materials in ways we don’t normally observe.
For instance, infrared light, with its lower energy, can pass through germanium, while visible light cannot. And while solid gold might seem impenetrable, its ultra-thin form becomes translucent, demonstrating the delicate dance of electromagnetic waves and electrons.
The implications of these discoveries aren’t just academic. From infrared cameras to optics used in space exploration, understanding these interactions has unlocked breakthroughs in technology. Has this article inspired you to craft something new? Or have you explored an effect similar to this? Let us know in the comments!
We usually take our germanium in the form of a diode. Or, maybe, a transistor.
There’s nothing odd about semiconductors being transparent to light. Electrically conductive materials absorb light well because they have “loose” electrons that can freely interact with the photons while insulating materials have their electrons bound to specific energy states that can only absorb photons of specific energies.
Consider: ordinary window glass is made of silicon. Just as well, germanium can be made into glass, and it is quite commonly used for infrared optics in that form.
Besides, Germanium is not considered a metal but a metalloid in the first place.
Depends on the reference. Several textbooks—including one I coauthored—eliminated “metalloid” entirely and classified those elements as either metals or nonmetals, depending on their physical and chemical properties. The classification of those elements is somewhat arbitrary anyway….some references classify polonium as a metal and astatine as a metalloid. It is a subject of some discussion and of opinion in chemistry.
Either way, Germanium is sitting on the fence between metals and non-metals however you categorize it, and as such it’s a somewhat poor electrical conductor, which explains why its electrons are not readily able to absorb some wavelengths of light but rather pass them through.
Silicon too is transparent in certain bands of IR light. Not the same as Germanium though, which is why the thermal camera with a Germanium lens will probably not see through a Silicon coin.
You’re one of those that loves it that Pluto has been classified as “NOT A PLANET” ….Party Pooper :-P
At least astronomers aren’t bickering about metalloids. Not hydrogen or helium? It’s a metal. Hydrogen, but squeezed really hard? Metal again.
And:
It’s not a rare phenomenon. An oil slick on water, or butterfly wings, exhibit the same effect: 100 nm is thinner than the wavelength of visible light, so it can pass through. The oil slick displays colors because the thickness of the film matches the wavelength of those colors.
Skin depth and thin film interference are definitely separable concepts. IIRC at 100nm thickness you’d get like 3% of the light passing thru gold just on the basis of skin depth, and if that’s the right figure, it can easily fit within the dynamic range of a camera and make it so that you can see both the original very bright object and the attenuated image onscreen at once without too much post processing. If it was thin film interference we were seeing, then the color would vary with angle due to the effective thickness changing.
The point is that the phenomenon has to do with the wavelength of the light matching or not matching some physical dimension of the object, which is a common thing you can see around you.
to be precise: germanium is not a metal, it is a semimetal.
It’s not that easy. A crystallographer would look at brillouin zones and the full band structure over Γ,L,X,U,K,Γ do describe solid state matter.
I see someone already submitted a non clickbaity title into DeArrow database:
“Demonstrations of Germanium’s Transparency to Infrared Light and Gold Leaf’s Slight Translucency”
Looked up DeArrow, didn’t know that existed!
Thanks!
(for anyone else: it replaces youtube thumbnails and titles with crowd sourced not clickbait ones)
Somewhat related: IR light does not go though ordinary window glass. Your TV remote control probably does not work anymore if you attempt to shine it though a window from outside. IR thermo camera’s therefore also do not have lenses from glass, but other materials, and those materials are usually opaque to “visible” light. Transparent materials are only transparent in some limited range of wave length, and those ranges are not the same.
I used to occasionally prank friends by turning off their televisions, satellite receivers etc from outside friend’s houses through the windows so it’s definitely not all glass, perhaps it just attenuates it
The IR leds in remote controls are just barely infrared. You can use glass lenses to take near-infrared photos, they work just fine.
They have lenses made of glass – just not glass made of silicon dioxide.
Awe jeese. I see some poo-pooing of this article and the comments poo-pooing it tend to have pretty weak descriptions of what is going on.
First lets talk about something, transmission of light through a medium other than vacuum is dependent on that materials thickness. That’s why Gold, though opaque at common thicknesses, can be transparent to some wavelengths of visible light when thin enough. Interestingly the angle of the light hitting the material and the wavelength itself really matters here, it usually appears red due to the promotion of electrons into a resonant mode called a surface plasmon. This is true for aluminum, copper, and many other materials.
Secondly, transmission of light through a medium depends on that materials composition (in an electrical sense). In bulk we typically call that a materials permittivity which is as you may have guessed analogous to a refractive index and it’s extinction coefficient. The extinction coefficient is the complex component to a refractive index which helps explain how photons are absorbed while passing through a material. So silicon dioxide based glass has a very different permittivity function than silicon metal. Silicon like Germanium is somewhat transparent in SWIR, but mostly opaque elsewhere. Silicon dioxide glass is mostly transparent in the visible range but more opaque in SWIR and IR depending on the wavelengths. These are material properties.
Lastly, there are a host of other reasons why a material may be opaque to light, such as incident angle, material geometry, polarization states, other resonances (eg: wave guides), temperature, etc.
This stuff is really cool and interesting and I see no reason to poo poo it at all.
Thus, the lunar astronauts could see through their gold plated visors.
Thanks for the information on light through metals! bookmarked for when I have a moment to give it a proper look and chase tangents.
I used the gold plated filters in my (arc) welding hood, some 30~odd years ago.
The color rendition was far better than the old green glass filters. I could run a couple of numbers darker rating because of being able to see things better.
The gold filter lenses were made on a dark brown plastic stock, but that plastic stock always turned cloudy after a few weeks/couple months of use.
Another thing was the gold was easily scratched and thus difficult to clean.
So you needed to keep one of the clear protective sheets mounted on both sides of the filter to keep it from getting dirty.
Plus you had make certain the gaskets around the hood opening were always in good condition or else the smoke would get on the filter lens and wiping it off generally meant a scratched gold layer.
Any scratches let the arc light through and thus into your eyes. Not good.
They should have used the motor cross lens protectors, cleverly stacked layers that easily peel off.
So that drivers can easily, quickly and single handedly remove a dirty sheet.
Coca cola is also transparent to IR, I remember the fad of removing the IR filter from Webcams.
As is Guinness beer.
near infrared, mind you. Water is annoyingly opaque beyond 1050 nm.
Its not only Germanium which is transparent in far infrared. Nowadays other cheaper materials are used as transparent windows or lenses aswell:
– Zinc-Selenide (corrosive)
– GASIR (non corrosive)
– Thermal transparent plastics
One of my favorite trade show demos is viewing through a 4-inch thick brick of pure silicon with an infrared camera.
If you have not seen a chunk of pure silicon in the flesh, it’s a an opaque dark grey, most certainly not visibly transparent. The fact this camera could see through that much solid dark material made a great demo.
(if you try it yourself, the devil is in the detail: Silicon has a huge refractive index, around 4, so reflection and scattering off the surface made this actually hard to pull off. The surfaces had to be flat, parallel and mirror-smooth, and stuff behind and beside the camera had to be dark in the infrared, otherwise their reflection dominated the image.)