Grey, dark blue, black or even orange, the lake regularly changes colour. This is certainly not due to its mood swings, but to 𝗽𝗵𝗲́𝗻𝗼𝗺𝗲̀𝗻𝗲𝘀 𝗽𝗵𝗼𝘁𝗼-𝗽𝗵𝘆𝘀𝗶𝗾𝘂𝗲𝘀. In fact, sunlight is composed of several wavelengths that make up the 𝗰𝗼𝘂𝗹𝗲𝘂𝗿𝘀 𝘃𝗶𝘀𝗶𝗯𝗹𝗲𝘀 spectrum: 𝗹'𝗮𝗿𝗰 𝗲𝗻 𝗰𝗶𝗲𝗹. All matter absorbs certain colours in order to reflect back to us the colours it does not absorb.
Water is completely transparent in small quantities, but it still absorbs sunlight, 𝘀𝗮𝘂𝗳 𝗹𝗲 𝗯𝗹𝗲𝘂. As a result, the greater its volume, the greater the number of molecules that make it up, allowing light to diffuse and reflect back to us the colour that is not absorbed: blue. This is why oceans, seas and all other large bodies of water are generally blue.
The lake is frequently tinted greenish blue or even turquoise, almost passing through yellow.
In freshwater bodies such as Lake Geneva, the water can be easily disturbed by algae, sediment and mud. These particles are responsible for changes in the lake's appearance, and their nature determines the colour we perceive. For example, algae that contain chlorophyll (a green pigment) give us a green colour. Conversely, the 𝗱𝗶𝘀𝗽𝗲𝗿𝘀𝗶𝗼𝗻 𝗱𝗲 𝗺𝗶𝗰𝗿𝗼𝗽𝗮𝗿𝘁𝗶𝗰𝘂𝗹𝗲𝘀 𝗰𝗮𝗹𝗰𝗮𝗶𝗿𝗲𝘀 very white during windy episodes interacts with the light to send back a turquoise blue, worthy of the Oeschinensee! (On the heights of Kandersteg, in the canton of Bern)
The surface of the water can change everything: small ripples in bright sunlight can create a sparkling lake, and when it's very calm you can even see a mirror effect, often sought after by photographers.
At dusk, when the lake takes on a reddish-orange hue as the sun sets, it simply diffuses the ambient light that could make you think Lake Geneva is on fire!
𝐒𝐨𝐮𝐫𝐜𝐞𝐬 :
- Conference given by Marie Perga, associate professor at the University of Lausanne.
