why does a ‘hudson’ flame glow red?

Posted on Categories color

blog post courtesy of Nicholas Bigelow, PhD, explaining the red flame of a ‘hudson’ glassybaby.

the ‘hudson’ glassybaby is made of an unusual material, known as dichroic, or two-colored, glass that appears blue when light is reflected off its surface, and red when light passes through it. the particular form of dichroic glass used in the ‘hudson’ is a colloid of glass with nanometer-scale metal particles (often simply called “nanoparticles”) suspended in it. this type of dichroic glass was known to the Romans (the Lycergus Cup being a famous example:, but it was not until the 1950s that the underlying mechanism was found.

the metal nanoparticles suspended in the glass are the key to the colors found in ‘hudson’. metals, as conductors of electricity, have electrons that are free to move about anywhere inside the metal. these conduction electrons form a gas of charged particles that lives inside the metal, known as a plasma. and, just like the air in a flute, the plasma in the metal has harmonic vibration modes, with the frequencies at which they exist dependent on both the size and shape of the nanoparticles and what metal they are made out of. these harmonic modes of the plasma are called plasma polaritons, or just “plasmons” for short.

unlike the musical frequencies of the flute, plasmons in metal nanoparticles exist at the optical frequencies of light, on the order of a thousand trillion cycles per second. and, because the electrons are charged particles, the plasmons interact with electromagnetic fields, such as light. so, when light of a frequency that the plasmons vibrate at strikes the metal nanoparticles, the plasmons are excited. the plasmons may then re-radiate the light back out (scattering), or they may convert the light to heat through resistance from the electrons moving about (absorbance), among other effects.

the metal nanoparticles found in ‘hudson’ are made of silver or gold, and are likely around 50 nanometers in diameter. at this size, the nanoparticles both scatter and absorb blue-green light. so when we see light reflected off the surface, it is the blue-green light that’s scattered back at us. and when we see light that has passed through the glass of a ‘hudson’, all the blue-green end of the spectrum is absorbed, making the light a fiery red.

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