I read it on Yahoo Answers (link below) that the power of lens changes when the lens is placed inside water. But then, if we consider the formula f=c/2 , the radius if curvature of the lens still remains same, then, how can the focal length change?
While considering the definition (point where light rays meet) we see that the focal length changes.
But while considering the formula (f=c/2) focal length doesn't change.
Can anyone please explain this and please do mention whether the focal length actually changes or not.
http://answers.yahoo.com/question/index?qid=20090217024946AAk1njB
Best Answer
The formula $$f=\frac{c}{2}$$ where $c$ is the radius of curvature is for mirrors, not lenses.
The reason you are getting a wrong result is that you are applying a formula designed for mirrors to a lens.
Incidentally, the formula $f=\frac{c}{2}$ is still valid for mirrors underwater.