What determines the frequency of an electromagnetic wave?

The frequency of an electromagnetic wave is determined by the source that produces the wave.

In more detail, electromagnetic waves are produced by the vibration of charged particles. This vibration creates a disturbance in the electric and magnetic fields, which then propagates through space as an electromagnetic wave. The frequency of this wave, which is the number of complete wave cycles per second, is determined by the rate at which the charged particle vibrates.

For example, in a radio transmitter, an alternating current is used to make electrons in the antenna vibrate. The rate at which these electrons vibrate determines the frequency of the radio waves produced. If the current alternates quickly, the electrons vibrate quickly and high-frequency waves are produced. If the current alternates slowly, the electrons vibrate slowly and low-frequency waves are produced.

Similarly, in a light bulb, the frequency of the light (which is also an electromagnetic wave) is determined by the energy levels of the electrons in the atoms of the filament. When an electron drops from a higher energy level to a lower one, it emits a photon of light. The energy difference between the two levels determines the frequency of the light emitted.

In summary, the frequency of an electromagnetic wave is determined by the source that produces the wave. Whether it's the vibration of electrons in a radio antenna or the energy levels of electrons in a light bulb, it's the characteristics of the source that set the frequency of the wave.