Focal length of concave lens experiment

[ash_nar]

Is there any way to find rough focal length of concave lens and convex mirror without using convex lens? Though I did the experiments(with object and image needles, not candle) very carefully with no parallax errors during my practical class, my readings are incorrect. I want to know if der s way to find out rough focal lengths of diverging lens n mirror in a simple way so that I can manipulate my readings accordingly during practical exam... Plzzzzzz help...

 

[Andy Resnick]

One easy way to get a rough estimate is to use either the sun or an overhead light as the source- the souce is then approximately at infinity. For a convex lens, the measurement is the straightforward- the distance from the lens to the focused spot is the focal length. For a concave lens, it's a little more difficult because the image is virtual. However, by making two measurements at different distances from the lens to calculate the divergence, you could calculate where the virtual spot is by geometry.

At least that's what I would do for a first approximation.