Seen under certain conditions, the Lycurgus cup looks green. When the light passes through the glass, it takes on a red hue. The object dates from the 4th century AD: its change of appearance therefore works without switch, without screen and without electrical device.
The British Museum guide explains this effect by the small amounts of gold and silver present in the glass. The decoration tells a transformation of another order: in the myth represented, King Lycurgus is imprisoned by a vine. The openwork relief and the behavior of the light thus give two very different ways of looking at the same object.
The surprise does not prove that the Roman glassmakers had our theories on matter. It bears witness to a remarkable technical result, the modern explanation of which should not be confused with their knowledge. The cup has retained its special color for a long time; what changes before our eyes is the light with which we look at it.
What we know
Real optical effect; ancient theoretical knowledge not inferred.
To go to the source
References consulted on September 21, 2026. Linked pages may be in another language.
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