# Quick lens problem, is f positive or negative

grandpa2390

## Homework Statement

the focal point is said to be real and positive for concave mirrors and virtual and negative for convex spherical mirrors. what if the problem gives you the focal point for a spherical convex as 15 cm. am I supposed to take it that the the focal point should be plotted one the positive side, or am I supposed to make it negative in my solution since I know that f is negative for convex mirrors?

none

## The Attempt at a Solution

it isn't that type of problem.

Mentor
Can you post the full problem statement? The context could be relevant.

grandpa2390
sry. it says "a convex spherical mirror has a focal length of 40.0 cm. find the image distance."

Mentor
This is clearly impossible without knowing the object distance.

grandpa2390
I didn't post the entire question, I just quote the part that was relevant. I'm sorry, the next line of the problem gives you object height and distance from the mirror. both of which are positive.

Homework Helper
Assume |f| = 0.4 m.

Puzzling why you should post #1 with 0.15 m and then later mention 0.4 m ?!

Mentor
I didn't post the entire question, I just quote the part that was relevant.
I still think it would be useful to see the full problem statement. This would make the question much clearer (it would also avoid the 0.15m / 0.4m confusion).

grandpa2390
Assume |f| = 0.4 m.

Puzzling why you should post #1 with 0.15 m and then later mention 0.4 m ?!
typo. I am looking at so many problems. I got the numbers mixed up when I reposted. it should be 15 cm

grandpa2390
I still think it would be useful to see the full problem statement. This would make the question much clearer (it would also avoid the 0.15m / 0.4m confusion).
is it possible for it to be positive. that's all I want to know. Is it possible for a convex spherical mirror to have a positive focal point? You don't have to tell me specifically for the problem. just in general. because the book seems to say that if p is positive f is negative and vice-versa

Homework Helper
the focal LENGTH for a convex mirror must be negative ... the focal POINT (signed distance from mirror to focussed image) can be either.

grandpa2390
Homework Helper
is it (f) possible for it to be positive. that's all I want to know. Is it possible for a convex spherical mirror to have a positive focal point?
Internal inconsistency: all I want to know => no need for another question.

that if p is positive f
You never told us what p is. The template may seem beyond your comprehension, bit it does say
Homework Statement
and its use is mandatory in PF, even for grandpas.

Make a drawing and post it.

grandpa2390
Internal inconsistency: all I want to know => no need for another question.

You never told us what p is. The template may seem beyond your comprehension, bit it does say and its use is mandatory in PF, even for grandpas.

Make a drawing and post it.

You know the template is not beyond my comprehension, and I asked the full question. and now I am sorry. The true question was table 34-1 in my textbook: Fundamentals of Physics, 9th edition extended. Where for each kind of mirror, you are to fill out the chart. I thought it would be easier to ask the question with numbers. I am sorry that my question went beyond your comprehension by turning the table into a concept question, but you obviously don't know the concept.

I am sincerely sorry that I asked the question, I realize that I should have just gone to my professor in the first place. It would have been much easier and saved a lot of time. I asked my professor the same way that I did here, and he was able to give me a straight answer. Without seeing any problems. All I had to do was ask, is the focal point positive or negative for a convex spherical mirror. Of course, when I posted here, I thought those who didn't know the answer would just keep silent. I certainly did not think they would attempt to mask their ignorance with an arrogant show of pedantry, and then insults at my intelligence.

@lightgrav I did "like" your post, but in light of the rudeness of last poster, I want to extended a written thank you for replying and giving me a straight and respectful response. You have my gratitude.

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Homework Helper
Wow, wow, lashing out, eh ? I sure know the template is not beyond your comprehension, just inserted a bit of irony. Sorry if that went awry. No need to get all excited (wouldn't expect that from a grandpa :) ).

There are lots of books titled Fundamentals of physics. Just like there are lots of points P. It's just that I don't know which one you mean.

Let's make peace. I haven't been accused of, well, what it says, and I didn't mean to insult. Just tease a little.

Curious what your professor said, by the way.

BvU -- soon to be one too..;)

grandpa2390
Wow, wow, lashing out, eh ? I sure know the template is not beyond your comprehension, just inserted a bit of irony. Sorry if that went awry. No need to get all excited (wouldn't expect that from a grandpa :) ).
You should expect a grandpa to be grouchy. Especially when he perceives insubordination from a young whippersnapper :)
There are lots of books titled Fundamentals of physics. Just like there are lots of points P. It's just that I don't know which one you mean.
you're right. I should have listed the author. Halliday and Resnick

Let's make peace. I haven't been accused of, well, what it says, and I didn't mean to insult. Just tease a little.

Curious what your professor said, by the way.

BvU -- soon to be one too..;)

My professor showed me in the textbook where it says (and I am quoting page 929) "Fundamentals of Physics: Extended 9th edition" by Halliday and Resnick (don't sue me)
To distinguish the actual focal point of a concave mirror from the per- ceived focal point of a convex mirror, the former is said to be a real focal point and the latter is said to be a virtual focal point. Moreover, the focal length f of a concave mirror is taken to be a positive quantity, and that of a convex mirror a negative quantity. For mirrors of both types, the focal length f is related to the radius of curvature r of the mirror by

f=1/2 * r

To find the focus, send in rays parallel to the central axis.

where r is positive for a concave mirror and negative for a convex mirror.

which makes perfect sense, and easy to remember when it is defined this way, since it is defined according to the axis and not just memorized. and one more detail. from what he said, the length (which could be positive or negative) is negative when it is on the same side as the virtual image.

so when drawing my ray diagram, 15 cm in front of the mirror is a positive focal length. 15 cm behind the mirror is like a... virtual focal length and therefore negative. but in the calculations, you use the negative focal length.

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Homework Helper
I'll take all that (young, and whippersnapper) as a compliment. You can add obnoxious, because I still think "Assume |f| = 0.4 m" in post #6 is reasonable advice: they tell you it's a convex mirror, and you knew from the beginning that " f is negative for convex mirrors ".

So the f in the

## Homework Equations

none
##{1\over o} + {1\over i} = {1\over f}##
is negative.

Good thing it all makes sense and you find it easy to remember. And as you also indicate, in a drawing it pops up almost by itself.

And yes, on a forum it's rather easy to feel insulted. Happens all the time. A little understanding goes a long way.
And when I said "soon to be one", I meant grandpa, not professor.

grandpa2390
I'll take all that (young, and whippersnapper) as a compliment. You can add obnoxious, because I still think "Assume |f| = 0.4 m" in post #6 is reasonable advice: they tell you it's a convex mirror, and you knew from the beginning that " f is negative for convex mirrors ".

So the f in the

## Homework Equations

none
##{1\over o} + {1\over i} = {1\over f}##
is negative.

Good thing it all makes sense and you find it easy to remember. And as you also indicate, in a drawing it pops up almost by itself.

And yes, on a forum it's rather easy to feel insulted. Happens all the time. A little understanding goes a long way.
And when I said "soon to be one", I meant grandpa, not professor.
I thought you meant professor. well gl. grandpaing is awesome