What happens when you sample too slowly
To accurately capture a signal, you must sample at more than twice the highest frequency present in the signal. The Nyquist frequency is half the sampling rate.
Aliasing occurs when the signal frequency exceeds the Nyquist frequency. The signal "folds back" and appears as a lower frequency - a completely different signal than the original!
Examples: Wagon wheels appearing to spin backwards in films, moiré patterns on screens, ultrasound artifacts in medical imaging.
Prevention: Use anti-aliasing filters before sampling to remove frequencies above Nyquist.