The distance between the optical center and the principal focus is called ____?

The distance between the optical center and the principal focus is called ____?

آپٹیکل سینٹر اور پرنسپل فوکس کے درمیان فاصلے کو ____ کہا جاتا ہے؟
Explanation
Focal length is the distance between the optical center of a lens or mirror and its principal focus.
It determines how strongly the lens or mirror converges or diverges light.