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A convex lens (of focal length 20 cm) and a concave mirror, having their principal axes along the same lines, are kept 80 cm apart from each other. The concave mirror is to the right of the convex lens. When an object is kept at a distance of 30 cm to the left of the convex lens, its image remains at the same position even if the concave mirror is removed. The maximum distance of the object for which this concave mirror, by itself would produce a virtual image would be : |
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Answer» 20 cm `(1)/(v)-(1)/(u)=(1)/(f)` `rArr""(1)/(v)-(1)/(-30)=(1)/(20)` `rArr""(1)/(v)=(1)/(20)-(1)/(30)rArr v=+60cm` `f=20cm`  It is give that location of image does not change even if MIRROR is removed. It means image formed by lens must be at the centre of CURVATURE of mirror as shown in the figure. Hence radius of curvature of mirror can be written as follows. `R=80-60=20cm`, So focal length of mirror becomes 10 cm. We should know that concave mirror forms virtual image when object is placed betweendistance of object from concave mirror to FORM virtual image is 10 cm. So option (b) is correct. |
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