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?
Best Answer
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.