Determining Focal Length of Convex Lens Without Apparatus

[xploitpatcher]

So I got a lab to plan and design an experiment to determine the focal length of a convex lens and involve the use of lens eq. and by means of plotting a graph...I have a basic rundown of what needs to be done and all but I am somewhat in a dilemma i don't have any access to any of the apparatus and I am clueless as to what values to make up...yes make up ma professor said it doesn't have to be precise but i want a rough estimate...HELP...my bad wrong thred

 

[Andy Resnick]

It's easy to get a rough estimate- here's two ways:

1) For indoors- use the lens to make an image of the ceiling lights onto the floor. When the image is in focus, the distance from the lens to the floor is the focal length.

2) For outdoors: use the lens to burn ants. When you are able to do that, the distance from the lens to the ground is the focal length.

This works because of the approximation that the source (in either case) is at infinity.