JAMES STEVENSON / SCIENCE PHOTO LIBRARY JAMES STEVENSON / SCIENCE PHOTO LIBRARY
A child using a thermochromic (or strip) thermometer. The strip thermometer has several dots on it which change colour at different temperatures. Each dot has a coating of micro- encapsulated liquid crystals. The rod-shaped molecules of the crystals are arranged rather like the steps of a spiral staircase. At room temperature, the spiral is in a compressed form and does not reflect light. As the temperature of the crystal increases, the spiral 'stretches' and begins to reflect light at precise wavelengths. Eventually the spiral becomes fully extended, reflecting blue light. The composition of the crystal determines the activation temperature.
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