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A student focused the image of a distant object using a device 'X' on a white screen 'S' as shown in the figure. If the distance of the screen from the device is 40 cm, select the correct statement about the device.
(a) The device X is a convex lens of focal length 20 cm.(b) The device X is a concave mirror of focal length 40 cm.(c) The device X is a convex mirror of radius of curvature 40 cm.(d) The device X is a convex lens of focal length 40 cm."
(d) The device X is a convex lens of focal length 40 cm.
Explanation
From the given figure, we can observe that light is coming from a distant object, and get refracted through the device 'X'. Also, the image formed on the white screen 'S' is highly diminished which indicates that the device 'X' should be a lens as refraction occurs only in lenses. The lens will be convex in nature, as it is producing the image on the screen (the image formed by a concave lens cannot be obtained on a screen).
We know that when an object is placed at distant or at infinity then the image formed by a convex lens is real, inverted, highly diminished, and at the focus $(F)$ behind the lens.
Thus, as the image is formed at the focus then the distance 40 cm between device 'X' and white screen 'S' 40 cm implies that it is the focal length of the lens.
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