Physics

Problem

A student attempts to find the image distance for a concave lens and records the following steps: Step I:

u = -18\,\text{cm}

f = -6\,\text{cm}

Step II: Uses the lens formula

\displaystyle \frac{1}{v} - \frac{1}{u} = \frac{1}{f}

Step III: Writes

\displaystyle \frac{1}{v} - \frac{1}{18} = -\frac{1}{6}

Step IV: Continues the calculation to obtain

v = -9\,\text{cm}

At which step does the first sign inconsistency with the Cartesian convention appear? a) Step II b) Step I c) Step III d) Step IV