What does photochromicity refer to in materials?

Photochromicity refers specifically to the property of materials that undergo a reversible change in color when exposed to light, particularly ultraviolet (UV) light. This phenomenon typically occurs due to the structural changes in the molecules within the material, which absorb photons from the light and transition into a different electronic state. When the light source is removed, the material returns to its original state, causing the color change to revert.

This characteristic is commonly seen in photochromic glasses and lenses, which darken when exposed to sunlight and clear up indoors, making them beneficial for eye protection against harmful UV radiation. The key aspect of photochromicity is its reversibility; the color change can occur multiple times without damaging the material, making it a sought-after quality in various applications, particularly in optical devices.

The other definitions provided do not accurately capture the essence of photochromicity. For instance, the absorption and release of heat relates more to thermal materials, while changes in shape under mechanical stress are linked to materials exhibiting plastic deformation. Lastly, electrical conduction in response to pressure corresponds to piezoresistive effects in certain materials, rather than color changes induced by light. Therefore, the definition that aligns with the concept of photochromicity is that it refers to a