The correct option is **B. 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 key concept here is that not all light can cause this emission; only light with a frequency above a certain threshold can do so. This threshold frequency is a critical value that depends on the material of the metal. 1. **Understanding the Photoelectric Effect**: - When light shines on a metal surface, it can be thought of as being made up of particles called photons. Each photon carries a certain amount of energy, which is directly proportional to its frequency (f) according to the equation: \[ E = hf \] where \(E\) is the energy of the photon, \(h\) is Planck's constant (\(6.626 \times 10^{-34} \, \text{Js}\)), and \(f\) is the frequency of the light. 2. **Work Function**: - The work function (\(\phi\)) is the minimum energy required to remove an electron from the surface of the metal. It is related to the threshold frequency (\(f_0\)) by the equation: \[ \phi = hf_0 \] - While the work function is an important concept, it is not the term used to describe the minimum frequency itself. 3. **Threshold Frequency**: - The threshold frequency (\(f_0\)) is defined as the minimum frequency of incident light required to emit electrons from the metal surface. If the frequency of the incoming light is less than this threshold frequency, no electrons will be emitted, regardless of the intensity of the light. - This is because the energy of the photons at frequencies below the threshold is insufficient to overcome the work function of the metal. 4. **Photon Energy**: - Photon energy refers to the energy carried by a single photon, which is calculated using the formula \(E = hf\). While this is related to the photoelectric effect, it does not specifically denote the minimum frequency required for electron emission. 5. **Excitation Energy**: - Excitation energy refers to the energy required to move an electron to a higher energy level within an atom, not necessarily leading to its emission from the surface. This term is not applicable in the context of the photoelectric effect. ### Why Other Options Are Incorrect - **A. Work function**: This term refers to the energy needed to remove an electron from the metal, not the frequency. It is related to the threshold frequency but is not the same. - **C. Photon energy**: This is a general term for the energy of a photon, which varies with frequency. It does not specifically refer to the minimum frequency needed for electron emission. - **D. Excitation energy**: This term is used in the context of moving electrons to higher energy states within an atom, not for the emission of electrons from a metal surface. ### Summary - The minimum frequency of light required to emit electrons from a metal surface is called the **threshold frequency**. - The threshold frequency is directly related to the work function of the metal. - Light with a frequency below the threshold will not cause electron emission, regardless of intensity. - The photoelectric effect demonstrates the particle nature of light, as photons must have sufficient energy to overcome the work function. This understanding is crucial for grasping the principles of quantum mechanics and the behavior of light in various materials.