(a) Out of the two lenses, one concave and the other convex, state which one will diverge a parallel beam of light falling on it. Draw a ray diagram to show the principal focus of the lens.

OR

(b) A ray of light after refraction from a convex lens emerges parallel to its principal axis.

(i) Draw a labelled ray diagram to show it.

(ii) In this case, the incident ray before refraction from the lens passes through a point on its principal axis. Name the point.

(a) As stated in the chapter: "a double concave lens... Such lenses diverge light rays... Such lenses are also called diverging lenses." A concave lens is thicker at the edges than at the middle, so a parallel beam incident on it spreads out (diverges) after refraction, appearing to come from the principal focus on the same side as the incident beam.

Ray diagram (concave lens, principal focus F):

(b) As per the chapter: "A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis." So if the emergent ray is parallel to the principal axis, the incident ray (before refraction) must have been directed through the principal focus F1 (the focus on the same side as the incident ray/object).

(i) Ray diagram: an incident ray from the object passing through F1, striking the convex lens, and emerging parallel to the principal axis on the other side.

(ii) The named point is the principal focus, F1, of the convex lens.