ANS:D - all the above.

1. When the thickness of the lens is small: The lens equation assumes thin lenses, meaning the thickness of the lens is negligible compared to the radii of curvature of its surfaces. This approximation allows for simplification of the optical calculations and is commonly used in basic lens analyses.
2. Only to conjugate distances along the principal axis: The lens equation relates the object distance (�u), the image distance (�v), and the focal length (�f) of a lens. While it's often used to compute conjugate distances along the principal axis, it can also be adapted for off-axis calculations using appropriate modifications and considerations.
3. The computed quantities are accurate enough for geometrical optics of simple distances: The lens equation is derived based on geometrical optics principles, which assume that light rays travel in straight lines and undergo simple reflections and refractions. For distances and configurations within the limits of geometrical optics, the lens equation provides accurate predictions of image formation and optical behavior.

• All the above.