 Solution

ID: past-exam-of-the-physics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2014/paper-1/a5/solution

For the real object and real image, take positive distances

u = f + u^{'}

and

v = f + v^{'}

uv = f (u + v) ⟹ (f + u^{'}) (f + v^{'}) = f (2 f + u^{'} + v^{'}) ⟹ u^{'} v^{'} = f^{2} .

u^{'}, v^{'}

refer to the adjacent focal points, not the lens plane.

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