Apparent Depth — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Apparent Depth Valid for paraxial rays (near-normal incidence) when an object situated in a denser medium is observed from a rarer medium. Consider a point object at real depth d. Draw two rays: one normal to the surface, one incident at small angle i. Apply Snell's Law at the interface: mu sin(i) = 1 sin(r) (assuming air). Use small angle approximation: sin(theta) approx tan(theta). Express tan(i) and tan(r) in terms of horizontal distance x and depths d real and d app. Substitute to get d app = d real / mu. Observer must be in the rarer medium (typically air). Angles of incidence must be small (paraxial approximation). The interface between media must be planar. Believing the formula applies to oblique viewing with large angles. Confusing this with the case where the observer is in the denser medium (where depth appears increased). Confusing Apparent Depth with Normal Shift (Shift = Real Depth - Apparent Depth).