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 30 cm, select the correct statement about the device X.; A. The device X is a concave mirror of focal length 15 cm; B. The device X is a concave mirror of focal length 30 cm.; C. The device X is a concave mirror of radius of curvature 30 cm.; D. The device X is a convex mirror of focal length 30 cm.(All India - 2013)

Answer 1

Explanation:

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 ...

Answer 2

Thus device X must be a convex lens of focal length 30 cm.

Explanation:

Question statement:

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 30 cm.

Solution:

A device X is placed behind the mirror therefore the image is focused on the screen placed behind the device. It is seen that the device X is a lens not a mirror.

When the object is placed at a certain distance then the convex lens always forms the image of the distant object at its focus. Thus device X must be a convex lens of focal length 30 cm.