SAMPLING THEOREM 

The sampling theorem specifies the minimum-sampling rate at which a continuous-time signal needs to be uniformly sampled so that the original signal can be completely recovered or reconstructed by these samples alone. This is usually referred to as Shannon's sampling theorem in the literature.

If a continuous time signal contains no frequency components higher than W hz, then it can be completely determined by uniform samples taken at a rate fs samples per second .The sampling theorem essentially says that a signal has to be sampled at least with twice the frequency of the original signal. Since signals and their respective speed can be easier expressed by frequencies, most explanations of artifacts are based on their representation in the frequency domain. The sampling frequency required by the sampling theorem is called the Nyquist frequency.