The focal length of a normal eye-lens is approximately

The distance between the eye lens and the retina in a normal human eye is approximately $2 \text{ cm}$.

For viewing distant objects, the object distance is $u = -\infty$.

Using the lens formula, $\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$

Substituting $v = 2 \text{ cm}$ and $u = -\infty$, we get $\dfrac{1}{2} - \dfrac{1}{-\infty} = \dfrac{1}{f}$

$f = 2 \text{ cm}$

Thus, the focal length of a normal eye-lens is approximately $2 \text{ cm}$.