The reason why we typically describe electromagnetic (EM) waves using their wavelength or frequency instead of their amplitude is rooted in the fundamental nature of EM waves and how they propagate.
An electromagnetic wave is a combination of electric and magnetic fields oscillating perpendicular to each other and perpendicular to the direction of wave propagation. The amplitude of an EM wave represents the maximum magnitude of the electric or magnetic field at any given point in space.
While the amplitude of an EM wave is certainly an important characteristic, it does not provide sufficient information to fully describe the wave. The wavelength and frequency of an EM wave, on the other hand, are directly related to each other and are fundamental properties that determine the nature of the wave.
The wavelength of an EM wave is the distance between two consecutive points that are in phase (e.g., two crests or two troughs). The frequency, on the other hand, represents the number of complete oscillations (cycles) that occur in one second. These two properties are related by the equation: wavelength = speed of light / frequency.
Describing EM waves in terms of wavelength or frequency has several advantages:
Standardized units: Wavelength is typically measured in meters (or some multiple/submultiple of it), and frequency is measured in hertz (Hz). Using these standardized units allows for consistent comparisons and calculations.
Relationship to propagation: The wavelength of an EM wave determines how it interacts with objects and propagates through different media. It helps us understand phenomena like diffraction, interference, and scattering.
Energy and intensity considerations: The intensity or energy carried by an EM wave is directly related to its amplitude. However, when discussing the energy per unit area (radiant flux density), which is often relevant in practical applications, it is more convenient to use concepts like irradiance or radiant power, which depend on both amplitude and frequency.
Spectral analysis: Analyzing EM waves in terms of frequency allows us to study the spectrum of electromagnetic radiation, which encompasses a broad range of frequencies. This is particularly important in fields like optics, radio communications, and astrophysics, where different regions of the EM spectrum have distinct behaviors and applications.
In summary, while the amplitude of an EM wave is important, describing waves in terms of wavelength or frequency provides more comprehensive and practical information about their nature, propagation, energy considerations, and spectral analysis.