That’s interesting. Obviously, you’d put a center dot to disambiguate millihertz from meter-hertz, but I can’t recall ever having learned a rule about that. So some combinations of units are inherently ambiguous?

Also: Hz/dpt.

[–] renzev 17 points 3 months ago* (last edited 3 months ago) (4 children)

I can't tell which unit is more cursed: millihertz or meter hertz. Surely, anything that could be measured in millihertz is more natural to measure as a period, or as revolutions per minute or something, right?

EDIT: Also, TIL about dpt. Thanks!

A dioptre (British spelling) or diopter (American spelling), symbol dpt, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dpt = 1 m^−1.

[–] [email protected] 7 points 3 months ago

Something that occurs once a hour has a frequency of 277.777... μHz

[–] [email protected] 5 points 3 months ago (1 children)

You know, I always figured optometry involved like, super complicated math and shit.

Turns out it's just basic arithmetic.

Kinda like programming, in a way.

[–] [email protected] 1 points 3 months ago (1 children)

Yeah, from my experience ordering glasses, it's mostly about making sure the lenses are aligned with my retinas to focus the light in the right spots. All the numbers are just a way to formalize those measurements so that the lens maker gets it right

[–] [email protected] 1 points 3 months ago

My last optometrist refused to tell me what my prescription was. I should have insisted, they've since closed up shop.

[–] [email protected] 3 points 3 months ago* (last edited 3 months ago) (1 children)

TIL about dpt

Tell me you don’t [Edit: need] glasses without telling me you don’t need glasses :D

[–] renzev 2 points 3 months ago

I actually do have glasses, I just never bothered learning about any of the technical details behind my lenses. Optometrist measured my eyes, I chose the cheapest frame the store offered, came back a week later to pick up the glasses and that's about it.

[–] Eheran 3 points 3 months ago (1 children)

Speed as meter per hertz is a rather odd case, like with a machine that goes in discrete steps.

In any case, I never use implied multiplication (and others) and always simply put everything where it should be.

[–] [email protected] 6 points 3 months ago (1 children)

Speed is metre hertz rather than metre per hertz. Metres per hertz would measure absement, which is a measure of how far away something is from a start point and for how long. So being twice as far away for half the time would be the same amount of absement.

[–] Eheran 2 points 3 months ago (1 children)

Yes, sorry for the little error, my point is that m*Hz it is not generally speed but only in specific cases. Hz is not just "per time" but "something occurring x times per time", like a frequency or revolutions. You can not use it for regular speed, it has to be a periodic event. That is how it is defined:

The hertz is defined as one per second for periodic events.

This is why we have Becquerels to count decay events, which are random and not periodic, but the unit is also 1/s. And for the same reason you can not use m*Bq as speed.

[–] [email protected] 2 points 3 months ago

I never knew that Hertz requires a periodic event. Thanks for that and the Becquerel explanation!