CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 26

(i) Any two: the object should be very far away (a distant tree or building), so that the rays from it reach the mirror nearly parallel; the object should be well illuminated (in sunlight) and have a clear outline, so that its image on the screen is bright and sharp; nothing should obstruct the path between the object and the mirror. (ii) He should measure the distance between the mirror and the screen on which the sharp image of the distant object is formed. Rays from a distant object are nearly parallel to the principal axis, and after reflection from a concave mirror they meet at its principal focus; so the sharp image forms at F, and the mirror-to-screen distance is the focal length.