# What is the minimal size of the spec of dust I can see with naked eye?

When sun directly shines thru the window - I can see small specs of dusts in the air. All specs have different sizes.

Assuming direct sun luminosity is 100kLux, spec is a perfect sphere, reflection is isotropic, what is the smallest spec of dust I can see from 20cm?

Could I see smaller particles if I increase light intensity?

What is the absolute minimum of the object size given infinitely powerful light source? Will it be the quantum physics that places limits?

• How good are your eyes? – RedPen Sep 1 '15 at 14:11
• I guess the limit is the point, where the speck of dust does not scatter more light than the surrounding air (then, even if you crank up the light intensity, the speck of dust will not shine out of the light scattered from the surrounding air). – Sebastian Riese Sep 1 '15 at 14:58
• It's better to consider the case where light shines through a small opening in curtains. In the ideal case, you are then totally dark adapted. If you assume that your eyesight is as good as that of Brian Skiff, you can see stars as faint as magnitude 8, which corresponds to $1.3\times 10^{-9}$ lux – Count Iblis Sep 1 '15 at 16:50