Question

Sitas power of her lens is -4D. The focal length and the type of lens will be______ . Convex lens of focal length 25cm . concave lens of focal length 25cm . comvex lens of focal length 40cm . concave lens of focal length 40cm

Answer 1

Concave lens of focal lenght 25 cm becoz :-
D(dioptre) = 1/focal length(in meters)
therefore, -4 = 1/f
thus , f = -1/4 (in m)
--> f = -0.25 m or -25 cm
and we know that focal length is neg. for concave lens.
therefore a lens of power -4D is a concave lens of focal lenght 0.25 m or 25cm.

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