Flic en Flac isolated beach after sunset. Long exposure photography with clouds in the west of Mauritius. To understand what a long exposure is in photography, you just have to imagine that the shutter (the curtains) remains open for periods of time called "long". It's a question of semantics. More seriously: the photo sensor is exposed to light for periods that vary from a few tenths of a second to several hours or more. Some people also talk about slow exposure. During this exposure, the sensor receives light and on the parts of the image that move in the frame, a kind of averaging takes place. This averaging reveals that dreamy or wispy look (some would say). So, a relatively simple principle: what is fixed in the image appears sharp, what moves appears blurred. This is why in this picture, the surface of the sea at Flic en Flac seems soft.