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In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

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Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#151

Post by shultays »

bqmjjx0kac wrote: post #4

> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule. How did he know that the film of oil was one molecule thick? It feels like a huge assumption to me, but maybe this blog post left something out.

An idea: if oil forms a two dimensional shape, ie a single layer of molecules, then adding 2x amount of oil would give you twice the area. If it is three dimensional, say oil makes a bubble, then it would look smaller.

Of course this also fails if the oil formed a disc of X layers of molecules

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#152

Post by zokier »

tech_ken wrote: post #29
bqmjjx0kac wrote: post #4

> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule. How did he know that the film of oil was one molecule thick? It feels like a huge assumption to me, but maybe this blog post left something out.

Blog post seems to have elided this point, but it did link the original paper which was quite short: https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lite... Rayleigh's experiment was actually trying to solve for the minimum thickness of oil required to stop some camphor shavings from moving around on the water. He never states it explicitly, but I think the assumption is that the minimum thickness required to…

Reading the paper, there is no mention of sizes of molecules. Did Rayleigh actually make the connection between film thickness and molecular size at some point? Or is that just modern retconning?

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#154

Post by Jeff_Brown »

bqmjjx0kac wrote: post #4

> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule. How did he know that the film of oil was one molecule thick? It feels like a huge assumption to me, but maybe this blog post left something out.

And even assuming it's one molecule thick, how did he know how tightly the oil molecules in that layer pack together?

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#155

Post by [deleted] »

wwarner wrote: post #50

In 1676 Roemer estimated the speed of light by timing the orbit of Jupiter’s moon Io, noting that as the Earth approached Jupiter, Io emerged from behind Jupiter a little earlier every day, and as the Earth traveled away from Jupiter it appeared a little later every day, with the time of day varying by 22 minutes over a year. Knowing the difference between the two distances, he reckoned that light travels that distan…

[deleted]

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#156

Post by bryanlarsen »

ummonk wrote: post #150
CamperBob2 wrote: post #101

Earlier quoted context omitted.

If that's true, then the ChatGPT answer isn't wrong, at least for the specific case of olive oil. I love how a few self-appointed guardians of hackerhood are able to declare certain resources as haram , even when they propose a plausible answer to a question that went unanswered more than once in the thread. Instead of embarking on a discussion that leaves us all a bit better informed, the literal messenger is attack…

No the idea that Rayleigh knew the film was one molecule thick, let alone why it was one molecule thick, is clearly made up. The paper makes it clear that he was reporting the thickness, and thought this might provide information on molecule size.

LOL, the first line of the ChatGPT response:

`Lord Rayleigh didn’t know a priori that surface tension would cause the oil film to be exactly one molecule thick, `

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#157

Post by bookofjoe »

augustusseizure wrote: post #53
stevenwoo wrote: post #31

Earlier quoted context omitted.

Did you get your physics education in high school or university? I only had to take one physics class in the USA at college for my major, quantum electrodynamics for electrical engineering but my professor wrote the textbook and I recall he went over each experiment starting from the fundamentals of our understanding of the basics of the atom, Newton's understanding of light at the time, double slit experiment, to Ma…

What’s the name of that textbook? It sounds really interesting. Isaac Asimov wrote a couple books that follow the narrative of science from the beginnings up until the 80s or so, which I highly recommend. One is called Atom and is more focused on how we got to our “present” understanding of particles. There’s also one that takes a broader view, it’s something like History of Science (? not at my bookshelf right now).…

Also, George Gamow's work is exemplary in making complex ideas understandable.

See, for example:

>One, Two, Three... Infinity: Facts and Speculations of Science (1947)

https://www.amazon.com/One-Two-Three-Infinity-Speculations/d...

PDF: https://archive.org/details/OneTwoThreeInfinity_158

..........................

>Thirty Years that Shook Physics: The Story of Quantum Theory (1966)

https://www.amazon.com/Thirty-Years-that-Shook-Physics/dp/04...

PDF: https://archive.org/details/ThirtyYearsThatShookPhysics-TheB...

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#158

Post by tech_ken »

zokier wrote: post #152
tech_ken wrote: post #29

Earlier quoted context omitted.

Blog post seems to have elided this point, but it did link the original paper which was quite short: https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lite... Rayleigh's experiment was actually trying to solve for the minimum thickness of oil required to stop some camphor shavings from moving around on the water. He never states it explicitly, but I think the assumption is that the minimum thickness required to…

Reading the paper, there is no mention of sizes of molecules. Did Rayleigh actually make the connection between film thickness and molecular size at some point? Or is that just modern retconning?

It's really (really) quick but the first line of the second para is:

> In view, however, of the great interest which attaches to the determination of molecular magnitudes, the matter seemed well worthy of investigation...

So it seems like his main goal was to understand the size of molecules via his film-thickness measurements

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#159

Post by tech_ken »

youainti wrote: post #72
tech_ken wrote: post #29

Earlier quoted context omitted.

Blog post seems to have elided this point, but it did link the original paper which was quite short: https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lite... Rayleigh's experiment was actually trying to solve for the minimum thickness of oil required to stop some camphor shavings from moving around on the water. He never states it explicitly, but I think the assumption is that the minimum thickness required to…

Camphor would release compounds that adjust the surface tension of water. So the oil would break that direct relationship.

Ahhh that's cool, thanks for clarifying
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