point source S is placed midway between two converging mirrors having equal focal length f as shown in the figure.The values of d for which only one image is formed.

4f

If the image is formed at the object it must be formed at the center of curvature C. since only one image is formed by two mirrors, the object should be placed at their common center of curvature as shown in the figure; the mirrors are the part of a sphere of radius 2f.

⇒ 2f + 2f = d

⇒ d = 4f.

Note: If the distance between the mirrors is 2f then also image will be found at the same position. So second value of d = 2f.