Saved Bookmarks
| 1. |
A point object is placed at a distance of 12 cm on the axis of a convex lens of focal length 10 cm. On the other side of the lens, a convex mirror is placed at a distance of 10 cm from the lens such that the image formed by the combination coincides with the object itself. What is the focal length of convex mirror? |
|
Answer» Solution :For convex lens, `(1)/(V)- (1)/(-12) = (1)/(10)` `i.e.,v=60 CM` i.e., in the absence of convex mirror, convex lens will form the image I, at a distance of 60 cm behind the lens. Since, the mirror is at a distance of 10 cm from the lens, I will be at a distance of 60 - 10 = 50 cm from the mirror, i.e., `MI_1 = 50 cm`.  Now as the final image `I_2` is FORMED at the OBJECT O itself, the rays after reflection from the mirror RETRACES its path, i.e., rays on the mirror are incident normally, i.e., I, is the centre of the mirror, so that `R=MI_1= 50 `cm and hence `F=(R//2) = (50//2) = 25 `cm |
|