Ways to measure the focal length of a convex lens with only natural sun light What are the common ways to measure the focal length of a convex lens with only natural sun light?
 A: Given that you use the tag "home-experiment" I will give an answer in that spirit.
Obviously you can use the convex lens to focus the sunlight onto a piece of paper - find the distance where the paper catches fire and that is your focal length. That's how I did it when I was 4. Shoe laces too - they are really stinky when you get the distance right. Wear sunglasses while you do this - looking at the focused image of the sun is brutal on your eyes.
You can also use the lens in a crude camera - a box with a pair of cardboard tubes that allow you to slide the lens in and out, and tracing paper at the back. Now point at a distant well (sun) lit object, and "focus". Measure the distance. You could even repeat for different distances and see whether the lensmakers formula holds:
$$\frac{1}{f}=\frac{1}{d}+\frac{1}{\ell}$$
$f$=focal length, $d$ = object distance, $\ell$ = distance from lens to focal plan.
This has the advantage that you have multiple measurements and can fit a straight line by plotting $\frac{1}{\ell}$ as a function of $\frac{1}{d}$, with the intersection with the Y axis corresponding to "infinity" ($\frac{1}{\infty}=0$).
