# Lenses and Mirrors

#### Aerospace

Stumped...lol..

A concave mirror has a focal length of 62.4 cm.
Determine the object position for which the resulting image is upright and four times the size of the object.

What formula do I use for this question? Related Introductory Physics Homework Help News on Phys.org

Homework Helper
h'/h = -q/p
1/p + 1/q = 1/f

#### Aerospace

haha...i already knew those formulas.
but what's stumping me is...i only know f and the magnitude -.-
is there something that i'm not seeing? i can be totally blind sometimes!

#### turin

Homework Helper
I don't understand. What is "-.-"? Try drawing a ray diagram to help you visualize. Beware of the sign conventions for the concave mirror (which I don't recall off the top of my head).

#### Soilwork

What is the answer meant to be?

#### Doc Al

Mentor
Aerospace said:
haha...i already knew those formulas.
but what's stumping me is...i only know f and the magnitude -.-
is there something that i'm not seeing? i can be totally blind sometimes!
You have all the info needed to solve this problem, you're just not putting things together. The fact that the image is upright means that the linear magnification is positive: in this case m = +4. Now use the definition of m to write the object distance (p) in terms of the image distance (q). Then when you plug into the mirror equation, you can solve for p.

#### Aerospace

yup because
m = distance of image/distance of object so we can say that m = 4 hence
di = 4 x do (where i and o represent image and object)

Then we can plug that into the equation for focal length which is
1/di + 1/do = 1/f
and we know the f, so we can solve for 'do' to get the answer :)

thanks very much.

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