**Correct Option: B. The ejection of electrons from a material when it is exposed to light of sufficient frequency.** ### Detailed Explanation: The photoelectric effect is a phenomenon observed when light (or electromagnetic radiation) strikes a material, typically a metal, and causes the emission of electrons from that material. This effect is crucial in understanding the particle nature of light and was famously explained by Albert Einstein in 1905, for which he received the Nobel Prize in Physics in 1921. #### Step-by-Step Breakdown: 1. **Understanding Light as Particles**: - Light can be thought of as being made up of particles called photons. Each photon carries a specific amount of energy that is directly proportional to its frequency (E = hf, where E is energy, h is Planck's constant, and f is frequency). 2. **Interaction with Electrons**: - When light of a certain frequency hits a material, the photons can transfer their energy to the electrons in the material. If the energy of the incoming photon is greater than the work function (the minimum energy required to remove an electron from the surface of the material), the electron can be ejected from the material. 3. **Threshold Frequency**: - There is a specific frequency of light, known as the threshold frequency, below which no electrons will be emitted regardless of the intensity of the light. This is a key aspect of the photoelectric effect, demonstrating that light must have a certain frequency to cause electron emission. 4. **Emission of Electrons**: - If the frequency of the incoming light is above the threshold frequency, electrons are emitted. The excess energy (energy of the photon minus the work function) is converted into the kinetic energy of the emitted electrons. 5. **Experimental Evidence**: - The photoelectric effect can be demonstrated experimentally. When ultraviolet light is shone on a metal surface, electrons are emitted, while visible light, even at high intensities, does not cause emission if it is below the threshold frequency. ### Why Other Options Are Incorrect: - **Option A: The emission of electrons from a material when it absorbs thermal energy.** - This option describes thermionic emission, where electrons are emitted due to thermal energy rather than light. The photoelectric effect specifically involves light (photons), not thermal energy. - **Option C: The conversion of light energy into electrical energy through a semiconductor.** - This option describes the photovoltaic effect, which is related but distinct from the photoelectric effect. The photovoltaic effect involves the generation of voltage and current in a material (like a solar cell) when exposed to light, but it does not specifically refer to the ejection of electrons from a material. - **Option D: The scattering of light off electrons in a vacuum.** - This option refers to Compton scattering, where photons collide with electrons and change direction. While it involves light and electrons, it does not describe the emission of electrons due to light absorption, which is the essence of the photoelectric effect. ### Formulas and Common Pitfalls: - **Key Formula**: \[ E = hf \] - 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. - **Common Pitfalls**: - Confusing the photoelectric effect with thermionic emission or the photovoltaic effect. - Forgetting that the photoelectric effect requires light of a certain frequency, not just intensity. - Misunderstanding the role of the work function in determining whether electrons will be emitted. ### Revision Summary: - The photoelectric effect involves the ejection of electrons from a material when exposed to light of sufficient frequency. - Light is made of photons, and each photon has energy proportional to its frequency. - There is a threshold frequency below which no electrons are emitted, regardless of light intensity. - The phenomenon demonstrates the particle nature of light and is distinct from thermionic emission and the photovoltaic effect.