When you multiply two numbers and the result equals one, those numbers are reciprocals.
Reciprocals are inversely related. When one goes up, the other goes down.
Imagine you have exactly one full glass of water to split between two glasses. Drag glass A up to pour more into it. Watch what happens to glass B.
No matter how you split it, when you multiply them together, the answer is always one. That is what makes them reciprocals.
Tap a number to see its reciprocal. Notice: they always multiply to one.
To find a reciprocal, just flip the fraction. The reciprocal of 2 (which is 2/1) is 1/2. Flip the top and bottom.
In ultrasound, period and frequency are reciprocals. They multiply to equal one, and they are inversely related.
When frequency increases, period decreases. They are reciprocals and inversely related. You will use this relationship throughout ultrasound physics.