Chapter 30, Problem 2A

To determine

To distinguish: focal point and focal plane.

Explanation of Solution

Introduction:

Focal point is a point on the principal axis of a lens on its either side. A lens has two focal points named first and second focal points respectively.

Focal point for a converging lens or convex lens is a point on the principal axis of the lens where the light rays being incident parallel to the principal axis of a lens, converges at this point after refraction.

Focal point for a diverging lens or concave lens is a point on the principal axis of the lens where the light rays appear to diverge after being incident parallel to the principal axis of the lens.

Focal plane is any plane drawn perpendicular to the focal points of a lens. For a converging lens, the image of a light ray incident parallel to the principal axis of the lens, the image converges through focal plane. And for a diverging lens, the image of a light ray incident parallel to the principal axis of the lens, the image appears to diverge through focal plane.

Conclusion:

Hence, focal point is a point on the principal axis of the lens where the light rays converge or appear to diverge while focal plane is a perpendicular line along the focal point of a lens.