# What is the smallest unit of measurement used that the eye can see?

• kyphysics
In summary, Kyphysics has not given much thought to the smallest unit of measurement that we can see and work with. It is a μm.

#### kyphysics

There are units of measurement smaller than our eyes can see, but that we use in science.

But, what is the smallest one that we can see and work with regularly?

I mean...on a traditional ruler, it'd be a millimeter, right? But, we can obviously go smaller - even if a ruler doesn't mark it. Anyone know the answer to this?

@kyphysics, have you given this any thought yourself?

The width of a human hair is between 17 μm to 181 μm.
A μm is 1/1000 of a mm.
These are easy to see and can be measured with a micrometer.

Large paramecia are maybe 40-80 μm wide and are easily visible in the right lighting conditions.

I have worked with both of these.

There are fluorescent microbeads of various sizes. Since they give off light, they may be visible as "there" but not properly resolved as individuals. However, you might be able to suck one up in a pipette. Depends what you mean by "work with".

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harborsparrow, Klystron, pinball1970 and 2 others
BillTre said:
@kyphysics,
A μm
How do you pronounce that unit of measure?

This is very subjective and based partly on peoples eyesight.

It would come up in reading the logarithmic scales of a slide rule where some people can estimate better than others.

With respect to the markings on a ruler, you would have the limitation of line thickness of the mark to consider. Its likely that 1/2 mm is the best one can do reliably unless the marking is finer.

On American standard you have 1/16 inch meaning your could estimate to 1/32 inch which is roughly 0.79 mm so I guess using mm markings you could get to 0.5mm

Lnewqban and BillTre
kyphysics said:
How do you pronounce that unit?
Its usually called a micron or a micrometer.
A million micrometers in a meter.

pinball1970, dextercioby and kyphysics
I can see the color difference between 600nm light and say 620 nm light. Does that count (think about a soap bubble). So 20 nm (.02 micrometers).

BillTre
BillTre said:
@kyphysics, have you given this any thought yourself?

hutchphd
A good ruler with a vernier scale, typically known as a vernier caliper, can can easily be read to 0.001 inch. Although my old eyes now need a magnifying glass to make that reading easily.

Wrichik Basu, jedishrfu, robphy and 2 others
jrmichler said:
A good ruler with a vernier scale, typically known as a vernier caliper, can can easily be read to 0.001 inch. Although my old eyes now need a magnifying glass to make that reading easily.
I take a photo with my phone, then zoom in.

Here's a virtual Vernier Caliper I made for my lab classes. (You can zoom-in on this one.)
https://www.geogebra.org/m/DemUu87n

Wrichik Basu and jrmichler
jrmichler said:
A good ruler with a vernier scale, typically known as a vernier caliper, can can easily be read to 0.001 inch.
Is a Vernier micrometer different in kind? They get you .0001
https://www.grainger.com/product/3D...PX5tgBeTzvAXXj-HmYxoC6qAQAvD_BwE&gclsrc=aw.ds
Maybe that's different ?
But there are a variety Moire pattern (vernier) scales that in principal can get you to any optical multiple. How about Vernier scale to read the Vernier scale to read the Vernier scale.
Probably there is an argument involving the Rayleigh limit for your eyeball somewhere in this.

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robphy said:
I take a photo with my phone, then zoom in.

Here's a virtual Vernier Caliper I made for my lab classes. (You can zoom-in on this one.)
https://www.geogebra.org/m/DemUu87n
View attachment 315247
Anyone have a "to internet scale/actual size" picture?

If you (anyone, I mean) could post it, that'd be visually helpful.

jedishrfu said:
This is very subjective and based partly on peoples eyesight.

It would come up in reading the logarithmic scales of a slide rule where some people can estimate better than others.

With respect to the markings on a ruler, you would have the limitation of line thickness of the mark to consider. Its likely that 1/2 mm is the best one can do reliably unless the marking is finer.

On American standard you have 1/16 inch meaning your could estimate to 1/32 inch which is roughly 0.79 mm so I guess using mm markings you could get to 0.5mm
I figured high school level or every day rulers wouldn't have super small units.

But thought a more "professional" or "science-y" version might (meaning they'd have very thin markings).

For most people, we don't need to get that precise in every day life. When's the last time you needed to measure anything less than a millimeter?

kyphysics said:
For most people, we don't need to get that precise in every day life. When's the last time you needed to measure anything less than a millimeter?
Why did you even start this thread?

Wrichik Basu, berkeman, phinds and 2 others
BillTre said:
Why did you even start this thread?
Why do you ask? It's a point of curiosity I have.

symbolipoint
kyphysics said:
Why do you ask? It's a point of curiosity I have.
Because the post BllTre called out:
kyphysics said:
For most people, we don't need to get that precise in every day life. When's the last time you needed to measure anything less than a millimeter?
seems to almost rebut the opening post.

Wrichik Basu and BillTre
Again: the fundamental limit of any optical device is the diffraction limit. Anything else depends upon your definitions. I believe the eye is close to the diffraction limit in its design.

DaveC426913 and BillTre
DaveC426913 said:
Because the post BllTre called out:

seems to almost rebut the opening post.
Nah. I separate practical use vs. knowledge. Most stuff we learn in school is never used in real life.

Although, you never know if you'll get that question on Jeopardy.

Hornbein and dextercioby
kyphysics said:
Nah. I separate practical use vs. knowledge. Most stuff we learn in school is never used in real life.
Yah. I think that's the rationale BillTre was looking for.

jedishrfu
kyphysics said:
For most people, we don't need to get that precise in every day life. When's the last time you needed to measure anything less than a millimeter?

Several times a week if not several times a day...

dextercioby, BillTre and kyphysics
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kyphysics said:
There are units of measurement smaller than our eyes can see, but that we use in science.

But, what is the smallest one that we can see and work with regularly?

I mean...on a traditional ruler, it'd be a millimeter, right?

Who are "we"? Do you mean the average person? Or collectively all people? If it's the former then the answer is probably "nothing" since most people don't measure anything on regular basis. But if you mean the latter then it's whatever you define as "seeing".

Such a vague question. You can answer it any way you want.

What I wonder is what qualifies as 'seeing'. Does a microscope count? I mean, it does nothing that your eyes don't already do. You can't disqualify a microscope simply due to it being a mechanical enhancement - unless you plan to disqualify glasses too.

DaveC426913 said:
What I wonder is what qualifies as 'seeing'. Does a microscope count? I mean, it does nothing that your eyes don't already do. You can't disqualify a microscope simply due to it being a mechanical enhancement - unless you plan to disqualify glasses too.
The microscope gives your eyes an enlarged look at stuff, no?

In that sense, it's not what the natural eye typically sees and wouldn't qualify.

Your eye "normally sees" light. The rest of the discussion is prattle. Really.

russ_watters
This seems like a good time to close the discussion. We've covered a lot of ground here and may fall off the world if we continue.

Thanks to everyone for contributing here.

Jedi

pinball1970, russ_watters and hutchphd