Question

A student determines the focal length of a device 'X' by focusing the image of a distant object on
a screen placed 20 cm from the device on the same side as the object. The device 'X' is
(a) Concave lens of focal length 10 cm
(b) Convex lens of focal length 20 cm
(c) Concave mirror of focal length 10 cm
(d) Concave mirror of focal length 20 cm

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Answer 1

Answer:

option d

Explanation:

there is a trick for this question that is whenever distant object given assume that light is coming from infinity

so rays from infinity meets at focus so screen is placed at focus hence focal lenght would be 20cm