The correct option is **A. Threshold frequency**. ### Detailed Explanation The photoelectric effect is a phenomenon where electrons are emitted from a metal surface when it is exposed to light of sufficient frequency. The minimum frequency of light required to eject electrons from the surface of a metal is known as the **threshold frequency**. #### Step-by-Step Explanation: 1. **Understanding the Photoelectric Effect**: - When light shines on a metal surface, it can transfer energy to the electrons in the metal. If the energy of the incoming light (photons) is high enough, it can overcome the attractive forces holding the electrons in the metal, allowing them to escape. 2. **Energy of Photons**: - The energy of a photon is given by the equation: \[ E = h \cdot f \] where: - \(E\) is the energy of the photon, - \(h\) is Planck's constant (\(6.626 \times 10^{-34} \, \text{Js}\)), - \(f\) is the frequency of the light. 3. **Work Function**: - Each metal has a specific energy threshold known as the **work function** (\(\phi\)), which is the minimum energy required to remove an electron from the surface of the metal. This energy is also related to the threshold frequency (\(f_0\)) by the equation: \[ \phi = h \cdot f_0 \] - Therefore, the threshold frequency can be expressed as: \[ f_0 = \frac{\phi}{h} \] 4. **Threshold Frequency**: - The threshold frequency is the minimum frequency of light that can provide enough energy (equal to or greater than the work function) to release an electron. If the frequency of the incoming light is below this threshold, no electrons will be emitted, regardless of the intensity of the light. 5. **Implications**: - If the frequency of the light is equal to or greater than the threshold frequency, electrons will be emitted. The excess energy (if any) is converted into kinetic energy of the emitted electrons. ### Why Other Options Are Incorrect: - **B. Cutoff frequency**: - This term is not commonly used in the context of the photoelectric effect. It may refer to a frequency limit in other contexts (like filters), but it does not specifically denote the minimum frequency required for electron emission. - **C. Resonance frequency**: - Resonance frequency refers to the frequency at which a system naturally oscillates. In the context of the photoelectric effect, it does not apply because the emission of electrons is not about resonance but about overcoming the work function. - **D. Emission frequency**: - This term is vague and does not specifically refer to the minimum frequency required for electron emission. It could imply any frequency at which electrons are emitted, but it does not capture the concept of the threshold frequency, which is a precise and critical value. ### Summary for Revision: - The **threshold frequency** is the minimum frequency of light needed to emit electrons from a metal surface in the photoelectric effect. - The energy of a photon is calculated using \(E = h \cdot f\), and the threshold frequency is related to the work function by \(f_0 = \frac{\phi}{h}\). - If the frequency of light is below the threshold frequency, no electrons will be emitted, regardless of light intensity. - Understanding the distinction between threshold frequency and other terms like cutoff or resonance frequency is crucial for mastering the photoelectric effect.