Question

A student has obtained an image of a well illuminated distant object on a screen to determine the focal length F1 of the given spherical mirror.The teacher then gave him another mirror of focal length, F2 and asked him to obtain a focussed image of the same object on the same screen.The student found that in order to focus the same object using the second mirror,he has to move the mirror away from the screen.From this observation it may be concluded that both the spherical mirrors given to the student were(select the correct option) A) Concave and F1F2 C)Convex and F1F2 Given answer is A, But please explain! ONE MORE, A student has obtained an image of a distant object on a screen to determine the focal lengthF1of the given lens.His teacher after checking the image, gave him another lens of focal length,F2and asked him to focus the same object on the same screen.The student found that in order to obtain a sharp image,he has to move the lens away from the screen.From this finding,we may conclude that both the lenses given to the student were A) Concave andF1F2 C)Convex and F1F2 Given answeris F1>F2 i.e., (D) Please clarify, As I am unable to understand the differences!

Answer 1

 in case 1---the answer given(A) is right beacause to get the real(on screen ) image of obj we need concave mirror and he has to move mirror away that means F1 is smaller than F2.(as a sharp image is obtained on focus only and when obj is at infinity images is on focus)
in case 2----the answer given(D) is right beacause to get the real(on screen ) image of obj we need convex lens and he has to move lens away that means F1 is smaller than F2.(as a sharp image is obtained on focus only and when obj is at infinity images is on focus)