Question

What is conjugate foci? Explain.

Answer 1

To understand and explain the concept of conjugate foci, we need to have the basic understanding of the concepts of a lens and concave mirror, about their principal axis, foci, conjugate points and how the images are formed through them.

Complete step-by-step answer:
The conjugate foci can be defined as the pair of two such points placed on the principal axis of any lens that, the object placed at the point, the image should be formed at a perfectly interchangeable point.
The lens system's object and image point will be known as conjugate points. Because all the light paths from the object to the image are reversible, an image will be created at the original object location if the object was positioned where the image is.

When under required condition the light diverges from a certain point on or to near the axis of the lens or spherical mirror, when focused at the other point. Then the point at which the light is converged and the source point are interchangeable or transposable.

Note: With the help of the concepts of conjugate foci we can find the focal length of a lens. If a converging lens is placed between an object and a screen, it is positioned so that the sharp image of the object is formed at the screen. Then $\dfrac{1}{f} = \dfrac{1}{p} + \dfrac{1}{i}$, where f is the focal length, p is the distance between the object and the lens (object distance) and i is the distance between the image and lens (lens distance).