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Visual science, period T (35/40) A time period T is the time taken when one complete cycle of vibration passes a point. As the frequency of a wave increases, the time period T of the wave goes down. The most common unit for measuring period is 'seconds' period is expressed in units of time such as seconds, hours, days or years. The period of orbit for the Earth to go all the way around the Sun is approximately 365 days.