*Question T6B07:
What causes a light-emitting diode (LED) to emit light?*  

*Correct Answer is  A: Forward current*  

### AI Response: (may contain errors)  

To understand what causes a light-emitting diode (LED) to emit light, we need to focus on the concept of forward current.

1. **Why the correct answer is correct:**
   - An LED emits light when it is forward-biased, meaning that a positive voltage is applied to the anode (the positive side) and a negative voltage to the cathode (the negative side). This allows current to flow through the diode. 
   - As electrons move from the n-type material to the p-type material within the LED, they recombine with holes, releasing energy in the form of photons (light). This process is known as electroluminescence.

2. **Why the other choices are incorrect:**
   - **B: Reverse current**: In reverse bias, the LED does not conduct current effectively, and thus does not emit light. Instead, it may lead to breakdown if the reverse voltage exceeds the diode's rating.
   - **C: Capacitively-coupled RF signal**: This type of signal does not provide the necessary forward bias to the LED. It may induce a small AC voltage but is not sufficient to cause light emission.
   - **D: Inductively-coupled RF signal**: Similar to the capacitive coupling, this method does not create the required forward current for the LED to emit light. It may induce a voltage but lacks the direct current flow needed for light emission.

3. **Useful formulas or rules of thumb:**
   - The forward voltage drop for most LEDs typically ranges from about 1.8V to 3.3V, depending on the color and type of LED. 
   - A simple rule of thumb is to ensure that the current flowing through the LED is within its rated specifications, usually around 20 mA for standard LEDs, to avoid damage and ensure proper light emission.

In summary, an LED emits light due to forward current, while reverse current and RF signals do not provide the necessary conditions for light emission.