Physics · Optics, Nature of Light and Optical Instruments

It is a fact.Albert A. Michelson, in 1893, measured the length of a standard meter in terms of the wavelength of red cadmium light. He used interferometry to determine the wavelength of light emitted by heated cadmium and defined the meter based on a specific multiple of this wavelength. This method provided a more precise and reproducible measurement compared to the physical platinum-iridium meter bar used previously

This option is incorrect. Green cadmium light – While cadmium emits green light as part of its spectrum, Michelson specifically used red cadmium light for its stability and ease of measurement.

This option is incorrect.Violet cadmium light – Violet light has a shorter wavelength, making it more challenging to measure with high accuracy using the interferometric techniques available at the time. 

This option is correct.Albert A. Michelson, in 1893, measured the length of a standard meter in terms of the wavelength of red cadmium light. He used interferometry to determine the wavelength of light emitted by heated cadmium and defined the meter based on a specific multiple of this wavelength. This method provided a more precise and reproducible measurement compared to the physical platinum-iridium meter bar used previously

This option is incorrect. Blue cadmium light – Although blue light is part of the cadmium spectrum, Michelson chose red because it was easier to work with and provided more reliable measurements.