Transition from Radio to Light: Understanding the Frequency Boundary

The line between radio and light often appears arbitrary, with a range of electromagnetic waves bridging the gap, including microwaves and infrared. This article explores the fundamental concepts behind this transition, specifically how frequency and technology differ in understanding the dividing lines between electronic and optical technologies. The key to this distinction lies in the understanding of Maxwell’s equations and the behavior of electromagnetic fields at different frequency ranges.

The Boundary Between Electronic and Optical Technologies

The fundamental boundary between electronic and optical technologies can be understood by examining the two types of currents present in electromagnetic technologies: conduction current and displacement current. According to Maxwell’s equations, conduction current is the physical flow of charged particles, while displacement current arises from a time-varying electric field. Electromagnetic technology inherently involves both forms of current, with displacement current being particularly significant in capacitors.

At lower frequencies, the displacement current is typically a parasitic component, meaning it is not the primary source of current flow and acts as a side effect. As the frequency increases, the fraction of displacement current grows until it becomes comparable to conduction current densities. At this point, it becomes challenging to use electronic technologies because the parasitic displacement currents can interfere with the desired signals.

Conversely, in optical technologies, displacement current is the dominant form of current. It plays a crucial role in electromagnetic-wave propagation and mediates optical emission and absorption processes. The quantum-mechanical description of these processes involves the electromagnetic vector potential, the second derivative of which gives the displacement current. In this context, conduction currents act as parasitics, dissipating power from the optical signal.

Therefore, the boundary between electronics and optics is the frequency at which typical conduction and displacement current densities become comparable. In practical terms, this boundary is around one terahertz, although the line is not strictly defined and can be somewhat higher.

Frequency and Technological Progress

The concept of “frequency” is fundamental to distinguishing between radio and light. Frequency refers to the rate at which a current changes direction. In the realm of radio, frequencies are those where the current changes direction slowly enough that electronics can switch the current at that frequency. This has evolved over time, from early radio frequencies of 100 kilohertz (AM) to modern FM and beyond, with current technology exploring frequencies in the terahertz range.

When discussing the boundary between radio and light, it is important to note the transition point. As technology progresses, radio frequencies are increasing, and light frequencies are decreasing, leading to an overlap. Lasers, for instance, can be considered as making visible radio waves by the frequency range discussed here. Traditionally, light was generated by thermal agitation, which results in a broad spectrum of frequencies with waves not in step and often lacking polarization. In contrast, radio waves are characterized by their consistent polarization.

The detection mechanisms for radio and light also differ. Electronic devices can measure the electric field as it changes, which I would classify as radio receivers. Light, on the other hand, is detected based on the energy it delivers, often by causing chemical changes in the eye, photographic film, or semiconductor energy barriers. Due to the higher frequencies of light, even small changes in energy are less noticeable, making it a “high” frequency technology compared to “low” frequency radio.

Technological advances have blurred the distinction once clear between radio and light. Early distinctions were based on the ranges of frequencies, but now, with radio frequencies reaching higher levels and light frequencies decreasing, the overlap is significant. This means both radio and light technologies are used in overlapping frequency ranges, making the traditional boundaries less distinct.