<p>A student performs a photoelectric experiment using a clean metal surface and notices the following observations:</p>

<p>1. Even when the intensity of light is increased, the emitted electrons do not gain any extra energy.</p>

<p>2. For a certain light frequency, the photoelectric current stops completely at a particular negative potential.</p>

<p>3. When the frequency of incident light is reduced below a certain limit, no electrons are emitted regardless of intensity.</p>

<p><strong>Based on these clues:</strong><br />
(a) Identify and define the physical quantities related to each of the above observations.<br />
(b) Write the relevant relation connecting these quantities with energy or voltage.<br />
(c) Mention their SI units.</p>