Eduprobe
Question:
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.
The device X is a concave mirror of radius of curvature 40 cm.
The device X is a convex lens of focal length 40 cm.
The device X is a convex lens of focal length 20 cm.
The device X is a concave mirror of focal length 40 cm.
Solution:
As the image is focused on the screen placed behind the device X, thus device X is a lens but cannot be a mirror. Also, a convex lens always forms the image of a distant object at its focus. Thus device X must be a convex lens of focal length 40 cm.