1. Is Dust Floating Outside the Light Beam Too?
In the morning, when sunlight streams through a gap in the curtains, dust flickers and glows inside the thin beam of light. Outside the beam, the area is dark, and nothing seems to be floating there. The key point to consider is that "not seeing" and "not existing" are different things. According to the explanation of the Tyndall effect, when light hits tiny particles, they scatter the light, making the path of the light visible [1]. The glowing part is simply the area where light is hitting the dust. It is more natural to think that dust outside the beam is not glowing because it is not hit by light, so the scattered light does not reach your eyes.
However, this is an explanation of how we see things, not a count of the dust in the room. Still, if which dust particles we can see depends on where the light hits, then moving the light should change which dust particles are visible. This point can be checked in the "Try It" section later. So, why does dust hit by light send light to our eyes when we are looking from the side?
2. Why Can We See Dust When Light Hits It From the Side?
Light travels in straight lines. If there were no dust, the light would pass straight through the beam and not reach the eyes of someone looking from the side. The only light that reaches your eyes is the light that has hit the dust and changed direction. This phenomenon is called scattering [1].
This means dust does not glow on its own. Instead, it acts as a tiny reflector that sends some of the light that hits it in a direction different from the one it came from. The path of the light appears to glow because it is a collection of these relay points. This phenomenon is named after the discoverer and is called the Tyndall effect. However, the Tyndall effect was originally studied in colloids, which are liquids with fine particles scattered in them. Some explanations state that beams of larger dust particles in the air are strictly treated as Mie scattering [2]. Calling dust beams the "Tyndall effect" is likely a common usage because the appearance is similar. So, why are the same beams are harder to see in a bright room during the day?
3. Tyndall's Observation: Why Did the Same Smoke Look Blue and Reddish?
According to the Kotobank explanation, Tyndall observed columns of smoke rising from a distant hut roof on a windless day. When the background was a dark pine forest, the smoke looked blue due to scattered light. When the background was a bright sky, the transmitted light looked reddish [1].
Even though it is the same smoke, the appearance changes depending on the background. This observation teaches us that scattered light is "visible against a dark background" [2]. Applying this to the light beams in a room, it explains why the beams stand out when you close the curtains, leaving only a gap for light to enter, and make the surroundings dark. The darker the background, the more the small amount of light returned by the dust can reach your eyes without being overwhelmed by the surroundings. When beams are hard to see in a bright daytime room, it is likely because the scattered light is buried by the brightness of the background. So, what color do the beams appear to be? Tyndall's smoke looked blue, but the beams in a room often look whitish. The difference lies in the size of the particles.
4. Why Do Dust Beams Often Look Whitish Instead of Blue?
Scattering by spherical particles that are about the same size as, or larger than, the wavelength of light is called Mie scattering [3]. Dust includes particles that are larger than the wavelength of light. Such particles are treated as Mie scattering, and the differences in color become less noticeable, so they can look whitish [2]. The exact size of the particles visible in the beams is not fixed.
When scattering happens with little color difference, light of various colors mixes and is returned, so it looks whitish. Cloud water droplets are also larger than the wavelength of visible light, so they scatter light almost evenly via Mie scattering and look white [2]. It is thought that the same type of scattering is involved in the whitish dust beams. The kind of scattering depends on how the particles’ size compares with the wavelength of light. If the particles are very small compared to the wavelength, it is Rayleigh scattering. If they are larger than a certain size compared to the wavelength, it is Mie scattering [4].
Another point is that in Mie scattering, as particles get larger, light is scattered more strongly forward, and scattering to the sides and back decreases [3]. Applying this to a room, it is possible that the brightness of the beam changes depending on the viewing angle. However, there are no Japanese materials in our sources that show how indoor dust looks from different angles. So, we will leave this as a possibility.
5. Is Dust Involved in Light Columns Through Cloud Gaps?
Columns of light shining through gaps in clouds are called crepuscular rays. NASA Earth Observatory explains that for a photo of light columns, dust floating in the atmosphere at the time of the photo made the light beams more clearly visible [5].
Although the scale is different, the beams in a room and the light columns in the sky are connected by the fact that particles scatter light and reveal the path. According to materials from the Ministry of the Environment, particles 10 micrometers or smaller have a low sinking speed and stay in the atmosphere for a relatively long time [6]. However, this is about outdoor air and does not show how long they float indoors. Thus, we see that beams are more visible when light hits them, there are particles to return the light, and the background is dark. So, how can you check this with your own eyes at home?
6. Try Changing the Background of a Flashlight Beam at Home
Try this at home: At night, make the room dark and look at the beam of light from a flashlight from the side. Do not point the light at people's eyes, and be careful of your footing. You do not need to stir up dust. Look at the light beam in front of a white paper and in front of a black cloth separately, and compare how the beam looks. Place the paper or cloth behind the beam to serve as the background. By changing the background, you can see whether the beam looks different, as in Tyndall’s observation of smoke. Try moving the flashlight slightly to see if the glowing particles change.
If you want to read more, Kotobank has explanations of the "Tyndall effect" and "light scattering" in Japanese [1][4]. The Japan Meteorological Agency's satellite center page has a story about identifying aerosols using Mie scattering [7]. This article summarizes why dust is visible only where light hits it. The detailed distribution of particles in a room is not covered in detail here.