Question

define the term principal focus of concave lens with the help of a diagram

Answer 1

Concave lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A concave lens is adiverging lens. When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus

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.