*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 (PV) cell is defined as the relative fraction of light that is converted to electrical current. This means that the correct answer is:

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

### Explanation of the Correct Answer:
- **Efficiency of a PV Cell**: This is a measure of how effectively a solar cell converts sunlight (photons) into usable electrical energy (electrons). It is typically expressed as a percentage, calculated by the ratio of the electrical output to the solar energy input.

### Why the Other Choices are Incorrect:
- **A: The output RF power divided by the input DC power**  
  - This choice refers to radio frequency (RF) systems, not photovoltaic cells. PV cells deal with direct current (DC) generated from light, not RF power.

- **B: The output in lumens divided by the input power in watts**  
  - Lumens measure light output, not electrical power. This choice confuses the concepts of light output and electrical efficiency, which are not directly related in the context of PV cells.

- **C: The open-circuit voltage divided by the short-circuit current under full illumination**  
  - This choice describes specific electrical characteristics of a solar cell but does not define efficiency. Open-circuit voltage and short-circuit current are important parameters, but they do not directly measure how much light is converted into electrical energy.

### Useful Formulas:
To calculate the efficiency of a photovoltaic cell, you can use the following formula:

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

Where:
- $P_{out}$ = electrical power output (in watts)
- $P_{in}$ = solar power input (in watts)

This formula helps quantify how well a solar cell converts sunlight into electricity, reinforcing the concept of efficiency in photovoltaic technology.