Question

with the help of a diagram explain the principle focus of a concave lens?

Answer 1

WHEN OBJECT IS AT INFINITY THE RAYS WHICH ARE AT UPPER PORTION GET REFRACTED OUTSIDE AS IN DIAGRAM AND SAME IS THE CASE OF LOWER RAYS BUT THEY ARE REFRACTED I OPPOSITE DIRECTION TO THOSE OF UPPER RAYS THE RAY PASSING THROUGH THE OPTICAL CENTER(o) IS NOT REFRACTED . A VIRTUAL POINT LIKE IMAGE IS FORMED ON SAME SIDE OF LENS THIS POINT IS SAID TO BE THE PRINCIPAL FOCUS AND THE DISTANCE BTW (o) AND (f) IS CALLED  FOCAL LENGTH . FOR DIAGRAM SEE IN ATTACHMENT