For a thin lens, the image formed by the first refracting surface acts as a virtual object for the second surface. Which additional approximation is used to combine the two spherical-surface refraction equations into the thin lens relation?   
AThe lens material has refractive index equal to that of the surrounding medium
BThe lens aperture is taken to be infinite
CThe lens is thin enough that $BI_1 = DI_1$
DThe object is always placed at infinity
Answer & Solution
Correct answer: C. The lens is thin enough that $BI_1 = DI_1$
In the thin lens approximation, the two refracting surfaces are very close to each other, so the intermediate image distance measured from the first surface is effectively the same as that measured from the second surface: $BI_1 = DI_1$. This allows the two interface equations to be added directly and leads to the thin lens relations. The object need not be at infinity except for defining the focal length.
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