It is therefore no longer selective, which explains the white colour of the clouds, with shades of grey depending on the amount of sunlight absorbed. Diffusion is then reduced to multiple reflections on the droplet surface. Water droplets suspended in clouds and fogs can reach 100 microns, much larger than the wavelength of light. At the scales of air molecules, or water molecules constituting ice, objects much smaller (nanometers) than the wavelengths of light (between 0.4 and 0.8 μm), scattering is predominant for short wavelengths, thus selecting the color blue. Bluish colours in an ice cave due to Rayleigh scattering. The diffusion of light depends on the size of the diffusing objects, read The colours of the sky.įigure 1.
In English, the distinction between this phenomenon and molecular diffusion is clear since this phenomenon is called scattering. On the other hand, in a transparent medium, such as air or ice, whose composition is not uniform, diffusion may occur. It is then not subject to any distribution. On the contrary, light can propagate in a vacuum. SOMMERIA Jöel, Joël Sommeria Author Encyclopedia Environment Research Director at the CNRS, Laboratory of Geophysical and Industrial Flows (LEGI).įor quantities such as the mass of a contaminant, heat or the amount of movement, the existence of a stirring material at microscopic scales is necessary for their diffusion.
MOREAU René, Professor Emeritus in Grenoble-INP, SIMaP Laboratory (Science and Engineering of Materials and Processes), member of the Academy of Sciences and the Academy of Technologies.
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