*Question E6F09:
What is the efficiency of a photovoltaic cell?*  

*Correct Answer is  D: The relative fraction of light that is converted to current*  

### AI Response: (may contain errors)  

The efficiency of a photovoltaic cell is a measure of how effectively it converts light energy into electrical energy. Let's break down the correct answer and why the other choices are incorrect:

### Correct Answer: D
- **The relative fraction of light that is converted to current**:  
  This is the definition of the efficiency of a photovoltaic cell. It describes how much of the incident light energy is converted into electrical energy (current). The efficiency is typically expressed as a percentage.

### Why the Other Choices are Incorrect:

- **Choice A: The output RF power divided by the input DC power**  
  This choice is incorrect because photovoltaic cells do not deal with RF (radio frequency) power. They convert light (solar energy) into DC (direct current) power, not RF power.

- **Choice B: The output in lumens divided by the input power in watts**  
  This choice is incorrect because lumens are a measure of light output, not electrical output. Photovoltaic cells convert light into electrical energy, not the other way around.

- **Choice C: The open-circuit voltage divided by the short-circuit current under full illumination**  
  This choice is incorrect because it describes a ratio of electrical parameters (voltage and current) rather than the conversion efficiency of light to electrical energy. While open-circuit voltage and short-circuit current are important parameters for characterizing a solar cell, they do not directly define efficiency.

### Useful Formula:
The efficiency ($\eta$) of a photovoltaic cell can be calculated using the formula:

$$
\eta = \frac{P_{\text{out}}}{P_{\text{in}}} \times 100\%
$$

where:
- $P_{\text{out}}$ is the electrical power output of the cell.
- $P_{\text{in}}$ is the incident light power on the cell.

This formula helps quantify how much of the incoming light energy is converted into usable electrical energy.