Physics
Define the term principal foci of a concave lens and show them with the help of proper diagrams.
Answer
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.
Related Questions
Draw a diagram to represent the second focus of a convex lens.
Draw a diagram to represent the second focus of a concave lens.
A ray of light incident at a point on the principal axis of a convex lens passes undeviated through the lens.
(a) What special name is given to this point on the principal axis?
(b) Draw a labelled diagram to support the answer in part (a).
Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams.