Geometrical OpticsHard
Question
An object is placed on the axis of a convex lens. Its image is formed 80 cm from the object. The magnification is 3. The focal length of the lens is :-
Options
A.15 cm
B.20 cm
C.40 cm
D.45 cm
Solution
Given (-u) + v = 80 cm ...(i)
and magnification M =
= 3 or v = - 3u ..(ii)
u is taken negative for the real and inverted image (let us think of it)
From eqs. (i) and (ii), we get
(-u) + (-3u) = 80
or u = - 20 cm and v = + 60 cm
using lens formula,
f = 15 cm
and magnification M =
= 3 or v = - 3u ..(ii)u is taken negative for the real and inverted image (let us think of it)
From eqs. (i) and (ii), we get
(-u) + (-3u) = 80
or u = - 20 cm and v = + 60 cm
using lens formula,
f = 15 cm
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