What is principal focus? Explain principal focus with a diagram.

When the parallel rays of light incident on a lens after refraction they meet at a point or appear to meet at a point on the principal axis is called principal focus. 

Focus is the point where light rays originating from a point on the object converge. The principal focus is defined as the point where a beam parallel to the principal axis appears to diverge converges from a point on the principal axis after passing through the lens.the focus for a beam of incident rays parallel to the axis.The principal focus is defined as the point where a beam parallel to the principal axis appears to diverge converges from a point on the principal axis after passing through the lens.

Principal focus - When rays of light parallel to the principal axis are. incident on a spherical mirror all the rays after reflection either converge. to a point (in concave mirror) or appear to diverge from a point (in. convex mirror) on the principle axis This point is called the principle. focus of the mirror.

We can consider the sun's rays as all being parallel so we know where the shadow lines project back will be the principal focal point. Make an allowance for the lens centre and if you measure the distance between the lens centre and the principal focal point, you get the principal focal length.rincipal focus is a point on the principal axis where the rays which are incident parallel to the principal axis meet after reflection. ... Focus is a point where rays parallel to each other meet. It is not necessary for the rays to be parallel to the principal axis.