A point object O is placed midway between two concave mirrors of equal focal length f, on their common axis, and the light follows the path shown in the figure. The final image is formed at the position of the object. The separation between the two mirrors is .

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(2) 2f. Given: two concave mirrors of equal focal length f facing each other, O midway between them, and the final image back at O. Idea: the figure shows the beam between the mirrors running parallel to the axis, and a ray leaves a concave mirror parallel to the axis only if it set out from that mirror's focus. So O must sit at the focus of the first mirror: with separation d, d/2 = f. That parallel beam then falls on the second mirror, which brings it to its own focus -- and since O is midway, that focus is O itself. Hence d = 2f. One other separation also makes object and image coincide, d = 4f, with O at the common centre of curvature so each mirror sends light straight back on itself; it is not among the choices here.

A point object O is placed midway between two concave mirrors of equal focal length f, on their common axis, and the light follows the path shown in the figure. The final image is formed at the position of the object. The separation between the two mirrors is ______.

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Why the other options are wrong