The number of vibrations per second of an electromagnetic wave is referred to as its

• A. Frequency
• B. Wavelength
• C. Amplitude
• D. Phase

Answer: (A)

Frequency is the rate at which the electromagnetic field oscillates per second, so it counts how many complete vibrations pass a given point each second. It’s measured in hertz. This tells you how often the wave cycles in time. Wavelength, on the other hand, is the spatial distance between successive peaks of the wave. Amplitude describes how strong the oscillation is (the field’s maximum strength), which relates to brightness but not to how often the cycle occurs. Phase describes the position within the cycle at a reference point. In vacuum, frequency and wavelength are linked by c = fλ, but the quantity asked about—the number of vibrations per second—is frequency.