# Physics

### Chapter : 4. Light Refraction

#### Principal foci and focal length

(v) Principal foci and focal length :
(A) First principal focus and first focal length :
It is a fixed point on the principal axis such that rays starting from this point (in convex lens) or appearing to go towards this point (concave lens), after refraction through the lens, become parallel to the principal axis. It is represented by F1.

Figure : Ray diagram showing first principal focus
(B) Second principal focus and second focal length :
It is a fixed point on the principal axis such that the light rays incident parallel to the principal axis, after refraction through the lens, either converge to this point (in convex lens) or appear to diverge from this point (in concave lens). It is denoted by F2.

Figure : Ray diagram showing second principal focus
If the medium on both sides of a lens is same, then the numerical values of the first and second focal lengths are equal. Thus
f = f'
(vi) (vi) Aperture :
It is the diameter of the circular boundary of the lens.