Clouds and foam appear white despite being made of transparent materials like water vapor and air because of how light scatters when it encounters tiny particles or bubbles. When light passes through these structures, it bounces off countless small interfaces between different materials, causing the light to scatter in all directions. This scattering affects all wavelengths of visible light equally, combining to produce the perception of white. The effect becomes noticeable at a macroscopic scale due to the sheer number of scattering points.
Steve Mould's explanation highlights how this phenomenon transforms seemingly clear substances into visibly opaque white masses. The video illustrates that transparency depends on uniformity and direct light transmission, while the irregular distribution of particles disrupts this, leading to diffuse reflection. This principle applies not only to clouds and soap foam but also to other natural and man-made materials that appear white. It reveals how microscopic interactions shape our everyday visual experience.