Re: How find focal length of lens at home?

Waffa wrote:
How can I find the focal length of lens using items I have at home?

The web pages I have seen either use an optical bench or show an arrow as a starting object but I can't project an image of an arrow into a lens.

Could I somehow use a bright light bulb and focus the light from that onto a *** of paper until the filament is clearly defined? Or will this give incomplete info because I am not able to compare the size of the projected image of a filament with the original?

I have several handheld magnifying lenses I want to compare with one another for power and also to compare with the dioptre strength in lenses found in off-the-shelf reading glasses.

The equation is

1/o + 1/i = 1/f

where o, i, and f refer to the distances from the lens to the object and the image, and the focal length. If the object is very far away (e.g. the Sun), the equation becomes

1/i = 1/f

Determining the magnification isn't needed. Creating an image of the filament would do just fine.

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