The image of a distant object on a screen 20 𝑐𝑚 from the device 'X’ was obtained by a student while determining the focal length of the device 'X’ by focusing on the same side as the object. The device 'X' is:

i. A concave lens of focal length 10 cm

ii. A convex lens of focal length 20 cm 

iii. A concave mirror of focal length 10 cm

iv. A concave mirror of focal length 20 cm

                                      (OR)

A light from a point source is incident on it can be made a parallel beam by which of the following?

i. Concave mirror as well as convex lens

ii. Convex mirror as well as the concave lens

iii. Two plane mirrors placed at 90 to each other

iv. Concave mirror as well as a concave lens

In the case of a concave mirror, the image will be formed on the same side on the screen at its focal point when the object is at a distance after reflection, all the rays will convert and meet at the principal focus. 

So, the focal length is 20 𝑐𝑚. 

Hence option (d) Concave mirror of focal length 20 𝑐𝑚, is correct

                                          (OR)

When the light rays from a point source are incident on the concave mirror and the convex lens, the emergent beams are parallel, and the image is formed at infinity. Convex mirrors and concave lenses, on the other hand, always create a virtual image. Emergent beams from these are not parallel but diverge when the object is kept in focus. 

Hence Option (d) concave mirror and a concave lens, is correct.