The smallest difference in wavelength that can be measured for electromagnetic radiation depends on various factors, including the precision and sensitivity of the measurement apparatus and the type of radiation being considered.
In general, the measurement of wavelength differences can be achieved with high precision using techniques such as interferometry. Interferometers exploit the interference patterns created by the superposition of waves to measure small differences in wavelength.
For visible light, interferometers can measure wavelength differences on the order of nanometers (10^(-9) meters) or even smaller. Advanced techniques and instruments can achieve sub-nanometer precision.
In other regions of the electromagnetic spectrum, such as radio waves or X-rays, the techniques and instruments used for measurement may differ. In some cases, the measurement of wavelength differences can be challenging due to technical limitations or noise sources.
It's important to note that the precision of the measurement is not solely determined by the nature of electromagnetic radiation but also relies on the capabilities of the measurement apparatus and techniques employed. As technology and measurement techniques continue to advance, the ability to measure increasingly smaller wavelength differences improves.