Which of the following facts about the photoelectric effect can be understood without invoking the quantum concept of light propagation

Options

A. The rate of photoelectrons emission, when they are emitted, increases with the intensity of light used
B. There is a threshold frequency, below which no photoelectrons are emitted, no matter how long the light is thrown on the metallic surface
C. Once the frequency of light is more than the threshold frequency, photoelectrons are emitted almost instantaneously, no matter how weak the light intensity is
D. For each frequency of light, exceeding the threshold frequency, there is a maximum kinetic energy of the emitted electrons

Answer

To solve the question regarding which fact about the photoelectric effect can be understood without invoking the quantum concept of light propagation, we will analyze each statement provided.

### Step 1: Analyze the Statements
1. **Statement 1**: The rate of photoelectron emission when they are emitted increases with the intensity of light.
- **Analysis**: This statement can be understood using classical physics. Increasing the intensity of light means more photons are hitting the surface, which can lead to more electrons being emitted. Thus, this statement does not require quantum concepts.

2. **Statement 2**: There is a threshold frequency below which no photoelectrons are emitted, no matter how long the light is thrown on the metallic surface.
- **Analysis**: This statement relies on the quantum concept of light, specifically the idea that light must have a minimum energy (related to frequency) to eject electrons. Therefore, this statement requires quantum concepts.

3. **Statement 3**: Once the frequency of the light is more than the threshold frequency, photoelectrons are emitted almost instantaneously, no matter how big the light is.
- **Analysis**: This statement also involves quantum concepts, as it refers to the relationship between frequency and the energy of photons. The instantaneous emission of electrons is a quantum phenomenon.

4. **Statement 4**: For each frequency of light exceeding the threshold frequency, there is a maximum kinetic energy of the emitted electrons.
- **Analysis**: This statement is based on the photoelectric equation, which is a quantum concept. It relates the energy of the incoming photons to the kinetic energy of the emitted electrons.

### Step 2: Conclusion
Based on the analysis, the only statement that can be understood without invoking the quantum concept of light propagation is:
- **Statement 1**: The rate of photoelectron emission when they are emitted increases with the intensity of light.

### Final Answer
The correct answer is **Statement 1**.

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