The correct option is **A. Threshold frequency**. ### Detailed Explanation The photoelectric effect is a phenomenon where electrons are ejected from a metal surface when it is exposed to light of sufficient frequency. The minimum frequency of light required to eject these electrons 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 is defined as: \[ f_0 = \frac{\phi}{h} \] - If the frequency of the incoming light is less than this threshold frequency, the photons do not have enough energy to eject electrons. 4. **Threshold Frequency**: - The threshold frequency is the minimum frequency of light that can cause the photoelectric effect. If the frequency of the light is equal to or greater than the threshold frequency, electrons will be emitted. If it is lower, no electrons will be emitted regardless of the intensity of the light. ### Why Other Options Are Incorrect: - **B. Critical frequency**: - This term is not commonly used in the context of the photoelectric effect. It may refer to other phenomena in physics, but it does not describe the minimum frequency needed to eject electrons from a metal. - **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 photoelectric effect is not about resonance but about overcoming the work function of the metal. - **D. Maximum frequency**: - This term is misleading in this context. The maximum frequency would imply a limit beyond which no electrons can be emitted, which is not the case. Instead, as frequency increases beyond the threshold frequency, more energy is available to the emitted electrons, potentially increasing their kinetic energy, but there is no defined "maximum frequency" for the photoelectric effect. ### Summary of Key Points: - The **threshold frequency** is the minimum frequency of light required to eject electrons from a metal surface in the photoelectric effect. - The energy of a photon is directly proportional to its frequency, and it must meet or exceed the work function of the metal to cause electron emission. - Other terms like critical frequency, resonance frequency, and maximum frequency do not accurately describe the concept of the minimum frequency needed for the photoelectric effect. This understanding of the threshold frequency is crucial for grasping the principles of the photoelectric effect and its implications in quantum physics.