Why can we see the dust particles in a narrow beam of light (and not in an all lighted area)?

Let us say that I am sitting in a room with all the drapes open. Bright sunlight is coming through the window. The whole room is brilliantly lighted. I will not be able to see the dust particles suspended in air.

Now, if I draw the drapes close, keeping a small slit open, allowing only a beam of sunlight to come in, I will readily see the suspended dust particles in that beam. The same thing will happen in a dark night with the beam of light from a handheld battery torch.

What will be the scientific explanation for this? I can not see the dust particles when I have more light. But when I actually reduce the light and there is only one narrow beam present, I can see those minuscule particles.

How does a narrow beam of light enable me to see those fine elements?

• For the same reason you can't see the stars during the day. Your eye can't adapt to the dynamic range needed so the stars or dust get washed out. They don't stand out above the noise floor. Commented Nov 11, 2013 at 3:09
• @BrandonEnright Could you please convert your comment to a full answer with sufficient explanations? I am not a physicist, so you will have be patient in explanations. I have got an engineering background though. Commented Nov 11, 2013 at 3:19

Your inability to see the dust until you narrow the slit has nothing to do with the narrowness of the beam but instead the dynamic range of light that your eye can see at one time.

A bit of searching turns up reports of a contrast ratio for you eye at one time as between 100:1 and 1000:1. This means if you're in a room with a range of brightness greater than about 100 to 1 the brightest things will all be washed out as white and the darkest things will all be essentially black. This is obvious in photos that are "backlit" like this one: These horses aren't black but because the ratio of the bright light to the dark horses exceeds the dynamic range of the camera the sky is washed out white and the horses are in silhouette.

Your eye can adjust over time to a huge range but it can't utilize the whole range all at once.

In the case of dust reflecting light, if you allow a lot of light into the room the relative brightness between the small amount of light the dust is reflecting and the rest of the illuminated room prevent you from seeing the dust.

This is fundamental to signal processing. Why can't you hear a whisper in a noisy room? The noise of the crowd obscures the whisper. The difference between the signal you're trying to pick up and the background noise is called the signal-to-noise ratio. In the case of dust, the light let into the room is scattered and reflected in the room and causes the room to be illuminated. This is the noise that obscures the signal from light reflected off of the dust.

• Wonderful explanation and answer! Commented Nov 12, 2013 at 5:25
• You can see horses by improving the dynamic range but you won't separate the whisper from nose. So, these are different things. You confuse the blindness and darkness. Yet, they are different things. Darkness cannot be an example of blindness.
– Val
Commented Nov 12, 2013 at 5:25

Our eyes become more sensitive to light in dark rooms, especially if we give them a few minutes to adjust. Also the illuminated dust in a beam of light shows up because of the contrast between it and the dark background when the room is dark.

The dust isn't really microscopic.

• Changed to fine from microscopic. Actually, I wanted to mean very small, and used microscopic in a rather loose manner. Commented Nov 11, 2013 at 2:30

I would partly disagree with other answers here, and say that it is not related to the dynamic range of our eyes, or our eyes adjusting to the light. This is a factor, true, but the main factor is the background contrast.

If you sit in your dark room, with a white piece of white paper in the sunbeam, you won't see the white fluff in front of the bright white paper. It's not a matter of your eyes adjusting - it's more a matter that the background is bright enough to hide the specks, so they don't stand out.

Conversely, if you open up your whole room to the sunlight, so that the specks are hard to see, then hold a piece of black cardboard up, you'll see the specs of dust floating in front of the cardboard, even though the rest of the room is bright, and your eyes are adjusted to the bright room.