Negative and Positive Signs for Lenses

[Gundam00]

I'm trying to figure out a way where I can remember which sides are negative and positive for lenses. Can anyone help me? I'd rather have a process where I can really understand as opposed to rote memorization. Thanks!

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[Cawolf]

Unfortunately it is just convention.

There is no explanation as to why one side is positive and one is negative, other than it makes the formulas work. That convention works with positive focal lengths in converging lenses, and the opposite in diverging, to make the thin lens formula work.

 

[justadream]

Rote memorization is best for this topic.

Honestly, just memorize the formula and apply it to basic situations (e.g., plug and chug).

 

[texan2414]

I wanna expand on that a little-

For converging systems (concave mirrors/convex lens)- the focal point and image distance will be positive.
For diverging systems (convex mirrors/concave lens)- the focal pt and image distance will be negative.

 

[Gauss44]

I wanna expand on that a little-

For converging systems (concave mirrors/convex lens)- the focal point and image distance will be positive.
For diverging systems (convex mirrors/concave lens)- the focal pt and image distance will be negative.

If I remember right, this means making the focal distance negative for diverging systems in those formulas? Example: 1/f=1/di + 1/do

 

[Cawolf]

Yes, by convention the focal distance, f, is written as negative in the thin lens equation; when dealing with diverging lenses.