Diffraction of polychromatic light through a circular orifice. In clouds, reflections and refractions are multiple, which blurs the separation of colors, and restores the white color of sunlight.įigure 3. Water drops or ice crystals can act as prisms, which leads to rainbows ( Spectacular Rainbows) or other atmospheric halos phenomena ( Atmospheric Halos). The different wavelengths of white light have different refractive angles, which leads to colour separation. This process is observed in a prism like the one in the vignette of the focus Deviation of light by a prism (link). At the interface between two media, part of the light is reflected, and the other part passes through the interface with a modified propagation direction: this is called refraction. For objects that are large in relation to wavelength, it is possible to reason in terms of light rays, which corresponds to geometric optics. These three types of diffusion correspond to different modes of interaction of light with the diffusing object. The central spot has the largest width L= 2Dθ, where the small angle θ is λ/a. The diffraction of a red light through a thin slit leads to a pattern of red spots separated by dark areas. Thus, this colour is typical of the blue line of the Vosges above coniferous forests emitting pollen and isoprene microbeads (originally a compound of natural rubber).įigure 2. On the other hand, at intermediate scales, close to the wavelengths of light, such as pollens, aerosols and fumes, Mie scattering imposes a slightly bluish greyish grey colour. 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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