In the context of electromagnetic waves, the theoretical limits for wavelength are as follows:
The longest possible wavelength in the electromagnetic spectrum is determined by the size of the observable universe, known as the Hubble length or Hubble scale. The Hubble length is estimated to be around 93 billion light-years or roughly 8.8 × 10^26 meters. However, it's important to note that this extreme length is far beyond the scales typically encountered in everyday physics and is more relevant in cosmological contexts.
On the other hand, the shortest possible wavelength is determined by the Planck length, which is derived from fundamental constants of nature, such as the speed of light, gravitational constant, and Planck's constant. The Planck length is estimated to be approximately 1.6 × 10^-35 meters. At such small scales, the concept of classical spacetime breaks down, and the effects of quantum gravity become significant.
It's worth mentioning that these extreme values, the Hubble length and the Planck length, are purely theoretical and are not typically encountered in practical situations. In everyday applications and within the known ranges of the electromagnetic spectrum, the wavelengths of electromagnetic waves span a wide range from very long radio waves (measured in meters) to very short gamma rays (measured in picometers).