Define the term principal foci of a concave lens and show them with the help of proper diagrams.

A light ray can enter a lens from either side, therefore, a lens has two principal foci, one on either side of the lens. These are known as first focal point( or first focus) F₁ and the second focal point ( or second focus ) F₂

For a concave lens, the first focal point is a point F₁ on the principal axis of a lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens.

For a concave lens, the second focal point is a point F₂ on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point.