It’s a commonly seen phenomena, dust particles sparkling in a narrow beam of light, but why does this happen?
Mostly, this is a result of our ability to perceive contrasts and the ability of the dust particles to reflect light. Let’s start with the latter point.
Light is reflected or absorbed by surfaces. We experience this daily as we move through the world and are blinded by light reflected off of a window but not by the building itself. How much light is reflected is a function of the albedo of the surface, or how reflective the surface is. Snow is more reflective than dirt because snow is white or light coloured and dirt is dark. Most surfaces will reflect some amount of light. Dust is reflecting light all the time but we don’t notice it unless the contrast is high enough. Which brings us to the second point.
Humans can only detect a specific image when there is enough contrast, that is the difference between the brightest object visible and the darkest object visible. This is an important topic when looking at accessibility. Individuals with low visual acuity, reduced visual sensitivity to contrasts, and colour deficiencies benefit from having text and images meet a minimum level of contrast. But how does this relate to seeing our dust sparkle in a beam of light?
Well, when we are in a brightly lit room there is dust floating in the air and the light is reflecting off of it, but because the overall lighting level is higher there isn’t enough contrast between the dust and the rest of the room. But, when the room is darker over all, with just a thin beam of light coming in, the contrast of the beam and the reflected light against the darker room is sufficient for us to see.