# Thread: focal point, negative or positive?

1. For example:

A vase is placed 1.0 m from a concave len. The vase is 35 cm high. The focal length of the len is 30 cm. Calculate the distance of the image from the lens.

how do you know when the focal point is negative or positive?

2.

3. Do you expect us to do all your homework?

Concave negative. Convex positive. ?????

4. this is not homework . Is it the same for mirrors?

5. Originally Posted by John Galt
Do you expect us to do all your homework?

Concave negative. Convex positive. ?????
what made you think it was homework? sorry that i didn't know that Concave is negative and Convex is positive.

6. Originally Posted by icu
Originally Posted by John Galt
Do you expect us to do all your homework?

Concave negative. Convex positive. ?????
what made you think it was homework? sorry that i didn't know that Concave is negative and Convex is positive.
Because it is phrased like a homework question.

I do not know if concave is negative, etc. That is why I placed five question marks after the suggestion. That is all it was. A suggestion. I haven't done optics for decades.

7. Originally Posted by John Galt
Originally Posted by icu
Originally Posted by John Galt
Do you expect us to do all your homework?

Concave negative. Convex positive. ?????
what made you think it was homework? sorry that i didn't know that Concave is negative and Convex is positive.
Because it is phrased like a homework question.

I do not know if concave is negative, etc. That is why I placed five question marks after the suggestion. That is all it was. A suggestion. I haven't done optics for decades.
I just picked a homework question as an example so it would be easier to understand the question i was asking.

8. I am not sure there is a once-for-all fixed rule like "convex positive, concave negative". If there is one, it's by pure convention.

If you don't know or use that convention (like I don't), decide for yourself, choose a coordinate system and see if the results make sense. If they don't, try the opposite sign.

Good luck, and keep us posted,
L.

9. ok, i think i got it.

Convex lens gives a positive and concave gives a negative.
Convex mirror gives a negative and concave gives a positive. :-D

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