In the context of electromagnetic waves, it is not accurate to describe them as having "positive" and "negative" frequencies simultaneously. The frequency of an electromagnetic wave represents the rate at which the wave's electric and magnetic fields oscillate per unit of time.
However, in some mathematical representations or signal processing techniques, the concept of positive and negative frequencies can be used to simplify calculations or analyze certain phenomena. This is commonly encountered in the field of Fourier analysis, where complex numbers and complex exponential functions are employed.
In Fourier analysis, a technique called the Fourier transform is used to decompose a complex signal, such as an electromagnetic wave, into a sum of sinusoidal components of different frequencies. In this mathematical framework, positive and negative frequencies are introduced as a convenient way to represent the different components.
Negative frequencies in Fourier analysis do not correspond to physically distinct entities but are rather mathematical constructs that help describe the relationship between the real and imaginary parts of complex signals. In practice, when dealing with real-world electromagnetic waves, only positive frequencies are considered.
Therefore, in the physical sense, an electromagnetic wave typically has a positive frequency that represents the rate of oscillation of its fields. The concept of negative frequencies is a mathematical tool used in certain contexts but does not correspond to physically meaningful phenomena in the context of electromagnetic waves.