My 4 year old asked me and I think it’s a rather good question that I would like to answer so she understands. Thanks 🙏🏻
In: 1926
It’s a bit hard to cover all the interactions in a way that a 4 year old would understand, but hopefully this will kind of make sense to the average person:
When light hits a material, two things happen. Some amount of light gets reflected, eg bounces off the material, and the remaining light is refracted, meaning it enters the material.
(The amount of light that is reflected vs refracted is determined by snells law, which requires the index of refraction of the two materials, eg air and water, or vacuum and air. Index of refraction can be wavelength dependent for some materials such as metal, meaning some metals reflect some colors more strongly than others)
As light travels through this material, some amount of it get absorbed. Some materials absorb a lot of light quickly (Eg solid surfaces such as rock, wood), some absorb slower such as leaves, grass, skin, murky liquids, and some absorb light very slowly such as pure water, air molecules, etc. (Metals absorb light immediately, they don’t have any diffuse/translucency) The further light travels through the medium, the more light gets absorbed.
The physical quantity that determines how much light is absorbed is called the extinction coefficient. This has been measured for a large number of materials. It can also be wavelength dependent, meaning some colors are absorbed more quickly. Pure water is a good example, it quickly absorbs red wavelengths, giving it an aqua color over a distance.
There is one other important component which is scattering. Pure water does not have any scattering, however it often has dissolved chemicals and small organic matter such as algae and plankton. As light transitions between the water medium and organic matter, it changes directions and non-green wavelengths are quickly absorbed by organic matter. This causes scattering which makes the medium appear murky or foggy. This is the same mechanism that causes fog to be hard to see through. Fog does not absorb much light, however when light hits it, it tends to scatter in all directions, this is determined by something called a phase function. Some mediums scatter light in many directions causing haze, such as fog or air, whereas others scatter most of the light forward, such as pure water. (Two types of common scattering are rayleigh scattering and mie scattering, these contribute to the color of the sky, and the hazy halo you see around the sun)
(Source: I have spent years working on computer graphics algorithms and have a good fundamental understanding of some of the equations and physics of light)
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