Question

What do you mean by focal plane of a lens ?

Answer 1

Hint The light rays coming from an object kept at infinity converge or diverge ( converge in case of convex lens and diverge in case of concave lens) and are focussed at point. The plane in which focus lies is called the focal plane .

Complete Step-by-step Solution
Let us see here how a convex lens focuses images.
Case I : When the light rays are parallel to principal axis –

Case II : When the light rays are not parallel to the principal axis-
     

As we can see in both of the above pictures , the light rays coming from infinity are converged at a point called focus . Focus can lie above or below or on the principal axis depending on the incoming rays. The plane which all the focuses are most likely to lie in is called the focal plane. The focal plane contains the focus of the lens as we see in the picture .
The focal plane is perpendicular to the principal axis of the lens . The distance of the focal plane from the lens is equal to the focal length of the lens .

Note We know that every lens has two focuses , hence the number of focal planes will also be two ( according to the geometry of the lens) .