Solving Inverted Image w/Concave Mirror: 22 cm, 44 cm, & Focal Length

[noobie!]

Homework Statement

An inverted image is magnified by 2 when the object is placed 22 cm in front of a concave mirror. Determine the image distance and the focal length of the mirror.

Homework Equations

don't have,sorry

The Attempt at a Solution

firstly,i use M= -di/do then i got di=-44cm ,then substitute in into the equation 1/f=1/do + 1/di ..1/f= 1/22 + (- 1/44) =44cm..did i get my steps correct?then i refer to answers,i noticed that their solution is 1/f= 1/22cm + 1/44cm without the negative..why could it be so?so,are my steps correct?the solution also stated image is real if its being describe upright..according to what I've read,it should be when a real image will describe as inverted image while virtual is to be described as upright and located behind the mirror,its so contradicting..so could you please help me to clear my doubts..thanks for your kind help!

 

[LowlyPion]

The negative sign in this equation is used as a convention. By convention, if the magnification is positive, the image is upright. If the magnification is negative, the image is inverted (upside down).

http://en.wikipedia.org/wiki/Concave_mirror#Analysis

I think by the statement of the problem your magnification is -2 to begin with.

Also you will be dealing with this case:
http://en.wikipedia.org/wiki/File:Concavemirror_raydiagram_2FE.svg

 

[noobie!]

http://en.wikipedia.org/wiki/Concave_mirror#Analysis

I think by the statement of the problem your magnification is -2 to begin with.

Also you will be dealing with this case:
http://en.wikipedia.org/wiki/File:Concavemirror_raydiagram_2FE.svg

oh..did you actually mean that convention merely a choice for me..so,if i place a either a negative sign or i don't will also be correct?..then the problem the magnification should be -2 instead of 2?if yes then i understand completely...?

 

[LowlyPion]

oh..did you actually mean that convention merely a choice for me..so,if i place a either a negative sign or i don't will also be correct?..then the problem the magnification should be -2 instead of 2?if yes then i understand completely...?

The sign is apparently a convention that indicates the case that you have.

What they gave you was that the magnification is 2. But inverted ... so it's -2.

The formula is m = - di/do = -2 ... so the signs cancel looks like to me.

2 = di/do

 

[noobie!]

The sign is apparently a convention that indicates the case that you have.

What they gave you was that the magnification is 2. But inverted ... so it's -2.

The formula is m = - di/do = -2 ... so the signs cancel looks like to me.

2 = di/do

ah..i got your point..understood..thanks for your kind help..thanks!