Wrong sign in Focal length parameter? about ray-optics HOT 9 CLOSED

I followed the sign convention of the General Physics textbook I used. I think it is also the sign convention used in most high schools and universities. For example:
https://www.physics.louisville.edu/cldavis/phys299/notes/lo_spmirror.html

from ray-optics.

In my text book, if rays come from left to right, distances are measured positive to the right of vertex, negative to the left (Spanish):
https://screenrec.com/share/J3bX4RSMGD

See this reference in English:
https://www.excellup.com/classten/scienceten/lightreflection4.aspx

And this video:
https://www.khanacademy.org/science/in-in-class10th-physics/in-in-10th-physics-light-reflection-refraction/in-in-sign-conventions/v/sign-conventions-for-mirror-lenses

Check video transcript ("negative focal length automatically means concave mirror, positive focal length implies convex..."):
https://screenrec.com/share/iMSL9ebwIK

from ray-optics.

I added a checkbox for ideal curved mirror for the convention you mentioned (the Cartesian Sign Convention). For ideal lens, I think the Cartesian Sign Convention gives the same sign as the original convention, so no change is made.

This option is off by default, since most of the General Physics textbooks in use still follow the old convention (as far as I know). And at least in Taiwan, the high school education also uses the old convention (see the sentence below equation 4-2 on page 10 of https://moodle.fg.tp.edu.tw/~tfgcoocs/blog/wp-content/uploads/2015/10/%E9%AB%98%E4%B8%89%E7%89%A9%E7%90%864-2%E7%90%83%E9%9D%A2%E9%8F%A1%E8%AA%B2%E6%9C%AC_%E7%A9%BA%E7%99%BD.pdf).

If the new convention is the default in some country/region and the locale for that country/region is added, this option can be on by default for that locale.

from ray-optics.

I understand now, thanks for the explanation. I think this solution is perfect, thank you!

Have you considered the same solution for ideal lens?
I've studied that a convergent lens has a negative focal length (or a positive image focal leght), whereas a divergent lens has a positive focal length (or a negative image focal leght)
https://screenrec.com/share/dRqZjfUxtl

from ray-optics.

I use Google Lens to translate the link, but I only see "image focal length", not "focal length". Is there a reference that defines the "focal length" as you mentioned?

from ray-optics.

you can find the definition of "focal length" and "image focal length" here:
https://www.fisicalab.com/apartado/lentes-delgadas

again sorry for the Spanish, go to section "Focos"
https://screenrec.com/share/2UO8QRmade

from ray-optics.

I checked this at Wikipedia:
https://en.wikipedia.org/wiki/Focal_length#General_optical_systems

"Back focal length or back focal distance is the distance from the vertex of the last optical surface of the system to the rear focal point (F′)."

They use the term "back focal length", but I prefer "image focal length" because in divergent lens F' is at the front.

I think we can avoid sign convention misunderstanding just with your solution above?

from ray-optics.

Note that in the "thin lens approximation" section of that wiki article, it only uses "focal length", and the sign convention is the same as the simulator. I think the distinction you mentioned is only for thick lens, where the optical system is regarded at two separate surfaces having their own focal lengths.

Therefore, in this simulator, only the "Spherical lens" tool may have this concern, not the ideal (thin) lens. Currently, the only number-parameter for that tool is the refraction index, so nothing need to be changed for now.

On the other hand, if the Spanish education system always uses that term, even for thin lens, then the Spanish for "image focal length" should be used for the Spanish locale of the simulator (which does not exist yet. It would be a great idea to contribute a new locale by translating index.html and simulator/locale/en.js, then open a pull request.)

from ray-optics.

ok, understood
thank you!

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