Register to reply 
Help Calculating focal length for magnifying glass 
Share this thread: 
#1
Dec3011, 04:40 PM

P: 6

So I bought a magnifying glass for a project and I need to find out the focal length of it in order to use it. I'm given the magnification of the converging lens, which is 5, but I don't any methods or how I could find out the focal length given only the magnification. I attempted to set distance object that I actually measured and put it into the magnification formula and then took the distance of the image and put it into the focal length formula. However, when I tested it out it didn't seem quite right. Are there anyways I could do this mathematically? If not how else could I do this?



#2
Dec3011, 05:49 PM

HW Helper
Thanks
P: 5,133

Focal length can be calculated knowing the radii of curvature of both lens surfaces. But radius of curvature is a difficult parameter to measure. If you focus the sun to a spot on paper, the distance from the centre of the lens to the burning paper is the focal length. Centre of the lens being a point inside the glass, roughly midway between the two lens surfaces.
In the absence of sun, try reading a newspaper with the lens. The distance from the page to the centre of the lens where you judge print to be sharpest is the focal length. 


#3
Dec3011, 06:09 PM

Mentor
P: 40,639




#4
Dec3011, 06:51 PM

P: 6

Help Calculating focal length for magnifying glass



#5
Dec3011, 06:56 PM

Mentor
P: 40,639

Are you familiar with the thin lense formula, which relates the focul distance to the object and image distances? 


#6
Dec3011, 06:57 PM

Mentor
P: 40,639

Here is a link to the thin lens equation info:
http://hyperphysics.phyastr.gsu.edu...pt/lenseq.html . 


#7
Dec3011, 07:00 PM

Mentor
P: 40,639




#8
Dec3011, 07:00 PM

P: 6

Oh okay
I am familiar with the thin lense equation Thanks! I'll try this out 


#9
Dec3011, 07:13 PM

P: 6




#10
Dec3011, 07:22 PM

Mentor
P: 40,639

So as a practical matter with this newspaper technique, you could look past the newspaper at something a long ways away, and then try to keep that same relaxed focus as you change your gaze to the newspaper as seen through the magnifying lens. Try several of the methods we've talked about in this thread, and see what values of focal length you get. Kind of fun experiments... 


#11
Dec3111, 01:54 AM

HW Helper
Thanks
P: 5,133

I think that's how it works, anyway. 


Register to reply 
Related Discussions  
Calculating focal length for aspherical lenses  General Physics  17  
Focal point of image in magnifying glass  Introductory Physics Homework  0  
Focal point of image in magnifying glass  Advanced Physics Homework  0  
Magnifying Glass & UV  General Physics  2  
Calculating Focal Length  Introductory Physics Homework  0 