What best describes a square wave?

• A. The rising and falling edges are vertical because they are instantaneous, and the high and low durations are equal.
• B. The waveform is a sine wave.
• C. The edges are curved and the durations vary.
• D. The waveform has exponential rise and fall.

Answer: (A)

A square wave is a periodic signal that sits at one voltage level for a time, then switches abruptly to a second level for the same amount of time, repeating this pattern. In the ideal case, the transitions are instantaneous, so the edges are vertical, not sloped. The high and low portions are equal in duration, giving a 50% duty cycle, which is a defining trait of a classic square wave.

This is why the description of vertical, instantaneous edges and equal high/low times fits best. Other descriptions describe different waveforms or non-ideal behavior: a sine wave smoothly rises and falls, curved transitions with varying widths don’t match the sharp, equal-on-each-half nature of a square wave, and exponential rise/fall corresponds to charging/discharging dynamics rather than instantaneous switching.