The cadmium red line refers to a specific spectral emission line of cadmium with a wavelength of 643.8496 nanometers (nm) in the visible light spectrum. This line is part of the emission spectrum of cadmium and was historically significant as a reproducible standard of length in metrology.
Historical
- Metrological Standard: In the late 19th and early 20th centuries, precise measurements of length were crucial for scientific and industrial progress. The cadmium red line was selected as a reference because it offered a stable and reproducible wavelength when cadmium was vaporized and excited in a controlled environment.
- Replacement of Physical Standards: This spectral line became an essential tool for defining units of length, complementing or replacing physical standards like the International Prototype Meter, which was a physical bar made of a platinum-iridium alloy.
Key Characteristics
- Wavelength: 643.8496 nm, located in the red portion of the visible spectrum.
- Stability: The wavelength of the cadmium red line is highly consistent when measured under controlled laboratory conditions, making it suitable for precise calibrations.
- Reproducibility: The emission line can be consistently reproduced using cadmium vapor in a spectrometer or interferometer.
Applications
- Spectroscopy: Used as a calibration standard for spectroscopic instruments.
- Metrology: Played a role in the precise determination of the meter before the adoption of modern definitions based on the speed of light.
- Scientific Research: Provided a benchmark for experiments requiring accurate wavelength measurements.
Modern Status
The cadmium red line has largely been replaced by modern standards in metrology, such as those based on the speed of light. Since 1983, the meter has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second, making it independent of any specific material or wavelength.
Despite this, the cadmium red line remains a notable historical reference in the evolution of precision measurement and the development of spectral standards in science and engineering. It serves as a reminder of the importance of spectroscopy in establishing reproducible and universal units of measurement.
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