In photography rules of composition, the use of rhythm is an important tool because rhythm is so fundamental to us all.
Our heartbeat for example is the single most powerful example of rhythm in our lives, and since the dawn of time humans have had a kind of sympathy and love of rhythm which they have expressed through music, poetry and art.
"What reinforces the content of a photograph is the sense of rhythm - the relationship between shapes and values." Henri Cartier-Bresson (1908-2004), French Photographer
In visual art, rhythm creates a sense of underlying harmony and structure, and is often employed by artists to enhance visual appeal. It is not always apparent immediately you see a picture but your eyes will nonetheless sense its presence.
Without rhythm, an image would be dull and uninteresting - imagine a picture of a flat sea with a clear sky above it. And now put some dolphins in your imaginary picture, leaping in graceful arcs above the surface. The rhythm of the curves would give a very pleasing sense of grace and beauty.
Above: Boats in Harbour (Dalian, China) by Anne Darling
Rhythm in an image can be said to be repetition of a shape or line in one of two ways: either the shape or line simply repeats exactly or the shape/line repeats progressively. Progressive rhythm occurs when the shape decreases or increases in size and the shape is scattered throughout the picture instead of being in sequence (as for example in the facade of a building).
The picture above is full of gentle curves, a good example of progressive rhythm. The curves in the mid ground are not too scattered and line up in a loose way with the horizontal. This gives a feeling of gentle, soothing movement. The curves in the foreground are more scattered and give a greater sense of movement bordering on the chaotic. Together they form an interesting whole.
Look for rhythm when you are out and about, it is everywhere around you. And if you can capture rhythm in your photographs, they will really stand out. Try it and see!