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A germanium coin can look opaque under visible light yet transmit infrared; a sufficiently thin gold layer can let some visible light through. The apparent paradox disappears once you ask which wavelengths are passing through—and how thick the material is.
Transparency depends on the wavelength
“Transparent” is not an all-or-nothing label for a material. A material may transmit one part of the light spectrum while absorbing or reflecting another. Visible light is the band our eyes detect; infrared lies beyond red and is invisible to us. Seeing a material as dark or opaque therefore does not tell you whether infrared can pass through it.
That distinction is the key to both the germanium and gold examples: the material, the light’s wavelength, and the material’s thickness all matter.
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In a demonstration reported by Heidi Ulrich for Hackaday on January 5, 2025, a video by The Action Lab shows infrared passing through a germanium coin even though visible light does not. The coin’s appearance under ordinary light is not a reliable guide to its behavior in infrared.
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This wavelength-specific behavior has a practical counterpart in optical engineering. Edmund Optics describes its germanium meniscus lens material as opaque in the visible, with a stated transmission range of 2–16 μm. The supplier lists infrared imaging and surveillance, remote sensing, and infrared spectroscopy among the applications for germanium optics. Those ranges and applications describe engineered optics, not a guarantee that every piece of germanium transmits equally across the whole infrared spectrum.
Commercial germanium optics are made for particular bands
Germanium components are specified for the wavelengths they are intended to handle. Edmund Optics lists germanium windows with anti-reflection coating options for 3–5 μm, 3–12 μm, and 8–12 μm bands. The supplier notes that germanium’s high refractive index can make an anti-reflection coating necessary for adequate transmission in the band of interest.
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- The 1N34A is a germanium signal diode known for its low forward voltage drop (~0.2–0.3 V) and high sensitivity, making it ideal for detecting weak signals. It handles small currents efficiently with low noise, suitable for RF detection, AM/FM demodulation, and low-level rectification. Operating efficiently up to several MHz, it offers faster switching than many silicon diodes of its time. Its compact glass package ensures stability and reliability in educational, hobbyist, and vintage electronics applications.
- Low Forward Voltage Drop Only about 0.2–0.3 V, much lower than a typical silicon diode (~0.7 V). This allows it to detect very weak signals without significant loss — perfect for crystal radios.
- High Sensitivity for RF Detection Excellent for AM/FM radio demodulation, signal detection, and low-level rectification. Can work effectively with microamp-level currents.
- Germanium Material Lower barrier potential than silicon → better performance at low voltages. Also tends to produce less noise in small-signal circuits.
- Fast Switching for Its Era Though not as fast as modern Schottky diodes, it was quick enough for radio frequencies in the MHz range.
Temperature also affects performance: the supplier says germanium transmission decreases as temperature increases and advises use below 100°C. These are design constraints for optical components, rather than properties to infer from the appearance of an uncoated coin.
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Gold is normally treated as an opaque metal in everyday settings, but a very thin layer can transmit some visible light. Hackaday’s account of the 2025 demonstration says the gold was reduced to 100 nanometers and identifies the layer as gold leaf. That figure belongs to the reported demonstration; it is not established as the standard thickness of commercially sold gold leaf or as a universal threshold for visible transmission.
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- 🚀Germanium (Ge) single crystal is a chemically inert material, its transmission spectral range of 2-12um, is a very commonly used infrared optical material, with higher height, good thermal conductivity, insoluble in water characteristics.
- 🚀Germanium single crystal has good mechanical properties and thermal conductivity, and the absorption at 10.6um is very small, which is the ideal material for C02 laser lens, window and output coupling mirror. Germanium (Ge) single crystal is also used as the base material for various infrared filters.
- 🚀The germanium window using DLC film technology is recommended for the infrared thermal imager, which not only improves the wear resistance of the window, but also reduces the thickness of the window and reduces the overall weight.
- 🚀Infrared germanium single crystal window is widely used in night vision, detection, monitoring and other equipment, as an observation window of infrared thermal phase infrared detection lens, with high transmittance, strong thermal stability, stable optical properties and other characteristics.
- 🚀The products can be divided into uncoated, double-sided AR film,Double-sided DLC film Or one side AR film and other side DLC film.
The account does not give a transmission percentage, and it does not show that every gold-leaf sheet will produce the same result. A craft sheet’s thickness and construction may differ, so the demonstration should not be taken as a promise that ordinary retail gold leaf will behave identically.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the two examples show
- Germanium: visible opacity can coexist with infrared transmission, which is why germanium is used in purpose-designed infrared optics.
- Thin gold: the Hackaday account reports visible-light transmission through a 100-nanometer layer in a demonstration, not through gold at arbitrary thickness.
- Both: transmission depends on wavelength and on the material’s thickness; for practical optics, conditions such as temperature and coatings matter too.
For an optical material, the useful question is not simply “Is it transparent?” It is “Transparent to which wavelengths, at what thickness, and under what conditions?”
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