Question

define principal focus of a concave lens and draw diagram

Answer 1

Principal Focus of concave lens is the point on the principle axis from which all parallel rays coming from infinity appear to diverge from after passing through the lens
Draw a concave lens.Draw Principal axis.Let incident ray come from left of lens parallel to each other. After reaching aperture let them diverge.Draw them backwards with dotted lines and you will see they meet at a point on principal axis .That's the principal focus.If you are yet confused you can search in Google for the diagram.Search this site:http://www.tutorvista.com/content/physics/physics-ii/light-refraction/terminology-optics.php
Hope it helps.Happy New yrr:)

Answer 2

Principal Focus of concave lens is the point on the principle axis from which all parallel rays coming from infinity appear to diverge from after passing through the lens
Draw a concave lens.Draw Principal axis.Let incident ray come from left of lens parallel to each other. After reaching aperture let them diverge.Draw them backwards with dotted lines and you will see they meet at a point on principal axis .That's the principal focus.If you are yet confused you can search in Google for the diagram.