The outcome of such a collision varies dramatically depending on the size of the particle and the colour of the light. Let’s imagine a ray of light travelling through empty space, suddenly colliding with a particle. What is the fate of a photon that hits a particle? To answer this question, we first need to redefine it in a more formal way. If you want to understand how, we’ll have to delve deeper into the mathematics of scattering. Mie scattering gives clouds their white colour. Rayleigh scattering causes the sky to be blue, and sunsets to be red. The latter, models how light reflects on much larger compounds that are suspended in the lower atmosphere, such as pollen, dust and pollutants. ![]() The former models how light is reflected by the oxygen and nitrogen molecules that make up most of the air. Those two mathematical tools allow predicting how light scatters on objects of different size. Most optical effects that planets exhibit can be reproduced by taking into consideration two different models: Rayleigh scattering and Mie scattering. This is why the scientific literature presents several models of scattering, each one designed to describe a subset of optical phenomena occurring under specific conditions. ![]() Both its density and composition change significantly as a function of the altitude, making it virtually impossible to come up with a “perfect” model. Part of the problem comes from the fact that the atmosphere is not a homogeneous medium. Modelling atmospheric scattering is, in fact, exceptionally difficult. The interaction between light and matter is extremely complex, and there is no easy way to fully describe it. ![]() If we want an atmospheric shader that looks good, we have to step up our Maths. What we have omitted, however, is the fact that a single equation will not yield believable results. In the previous tutorial, we have derived an equation that provides a good framework to approximate atmospheric scattering in a shader.
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