The photoelectric emission from a surface starts only when the light incident upon the surface has certain minimum:

Options

A. (a) intensity
B. (b) wavelength
C. (c) Frequency
D. (d) velocity

Answer

To solve the question regarding the photoelectric emission from a surface, we need to understand the concept of the photoelectric effect and the factors that influence it.

### Step-by-Step Solution:

1. **Understanding Photoelectric Emission**:
The photoelectric effect occurs when light (or electromagnetic radiation) hits a surface and causes the emission of electrons from that surface. For this emission to occur, the incident light must have sufficient energy.

**Hint**: Recall that the photoelectric effect involves the interaction of light with electrons in a material.

2. **Minimum Energy Requirement**:
The energy of the incident light is given by the equation \( E = h\nu \), where \( E \) is the energy, \( h \) is Planck's constant, and \( \nu \) is the frequency of the light. For electrons to be emitted from the surface, the energy of the incident light must be greater than or equal to a certain minimum energy, denoted as \( E_0 \).

**Hint**: Remember that energy is directly related to frequency; higher frequency means higher energy.

3. **Threshold Frequency**:
The minimum frequency required for photoelectric emission to occur is known as the threshold frequency (\( \nu_0 \)). This frequency corresponds to the minimum energy needed to release an electron from the surface of the material.

**Hint**: Think about how frequency and energy are connected in the context of the photoelectric effect.

4. **Analyzing the Options**:
- **Intensity of Light**: While the intensity of light affects the number of electrons emitted (and thus the current), it does not affect whether emission occurs or not. Therefore, this option is incorrect.
- **Wavelength**: Wavelength is inversely related to frequency. Since the question asks for the minimum condition for emission, wavelength does not directly determine if emission occurs. Hence, this option is also incorrect.
- **Frequency**: This is the correct option because the photoelectric emission starts only when the frequency of the incident light is at least equal to the threshold frequency.
- **Velocity**: The velocity of emitted electrons is determined by the energy of the incident light, which is related to frequency, but it is not the minimum condition for emission. Thus, this option is incorrect.

5. **Conclusion**:
The photoelectric emission from a surface starts only when the light incident upon the surface has a certain minimum frequency, which is the threshold frequency.

### Final Answer:
The correct answer is **C) Frequency**.