1. A vacuum photocell is connected to in series with a power supply, a variable resistor, and a sensitive ammeter. A voltmeter is connected in parallel across the photocell.

Monochromatic light of frequency $f$ illuminates the cathode, which has a threshold frequency ${f}_{0}$. Photoelectrons are emitted and collected by the anode. A photocurrent is measured by the ammeter.

The potential difference across the photocell, $V$, is increased until the ammeter reads zero.

Which of the equations below does not correctly relate the potential $V$at which the current decreases to zero to the threshold frequency ${f}_{0}$ and frequency $f$?