Defining Fluorescence
Fluorescence is a type of luminescence where a substance absorbs light or other electromagnetic radiation and then re-emits light almost immediately. The re-emitted light typically has a longer wavelength, and therefore lower energy, than the absorbed radiation, meaning it appears as a different color (e.g., absorbing UV light and emitting visible blue light).
How Fluorescence Works
The process begins when an electron in a fluorescent molecule absorbs a photon, gaining energy and jumping to a higher energy state. This excited state is unstable, and the electron quickly loses some energy through vibrational relaxation, usually as heat, moving to a slightly lower excited state. It then returns to its ground state by emitting a photon of light, which is the fluorescence observed. This emission happens very rapidly, typically within nanoseconds of absorption.
A Practical Example
A common example of fluorescence is the glow from a highlighter pen under a black light (UV light). The ink in the highlighter contains fluorescent dyes that absorb the invisible ultraviolet light and then re-emit it as a brighter, visible light, making the highlighted text appear to glow. Another example is the brightening agent used in laundry detergents, which makes white clothes appear whiter and brighter by converting UV light into visible blue light.
Importance and Applications
Fluorescence is crucial in many scientific and technological fields. In biology and medicine, fluorescent dyes are used to label cells, proteins, and DNA for imaging and diagnosis, such as in fluorescent microscopy and PCR. It's also used in forensics to detect fingerprints or bodily fluids, in security features for currency and documents, and in energy-efficient fluorescent light bulbs which convert UV light into visible light.