Focal length is the distance from the center of a lens to its focal point, where parallel light rays converge or appear to diverge. It determines the converging or diverging power of the lens.
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The focal length (f) is positive for convex lenses and negative for concave lenses.
Focal length is measured in meters (m) in the SI unit system.
The lens formula relates focal length (f), object distance (u), and image distance (v): $$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$.
A shorter focal length indicates a stronger converging or diverging effect of the lens.
The magnification (M) of a lens can be calculated using the formula $$M = \frac{v}{u}$$, where v is the image distance and u is the object distance.