I must keep the keep the distance, I have to keep the distance of the object will keep between f and 2f means, between a focal point and the at the point of ready centre of curvature 2f c.
मुझे वस्तु की दूरी f और 2f के बीच में रखनी होगी मतलब फोकल बिंदु और वक्रता केंद्र 2f के बीच में।