An air core coil and an electric bulb are connected in series with an AC source. If an iron rod is put in the coil, then the intensity of light of the bulb will

Options

A. remains same
B. increases
C. decrease
D. first decrease then increase

Explanation

To solve the problem step by step, we need to analyze the effect of inserting an iron rod into an air core coil connected in series with an electric bulb and an AC source.

### Step-by-Step Solution:

1. **Understanding the Circuit**:
- We have an air core coil and an electric bulb connected in series with an AC source. The current flowing through the circuit will determine the intensity of the light emitted by the bulb.

**Hint**: Remember that in a series circuit, the same current flows through all components.

2. **Effect of Inserting the Iron Rod**:
- When an iron rod is inserted into the coil, it changes the magnetic properties of the coil. Specifically, it increases the inductance of the coil because iron is a ferromagnetic material.

**Hint**: Consider how the presence of a ferromagnetic material affects the inductance of a coil.

3. **Inductance and Impedance**:
- The increase in inductance leads to an increase in the impedance (Z) of the circuit. The impedance in an AC circuit is given by the formula:
\[ Z = \sqrt{R^2 + (X_L)^2} \]
where \(X_L\) is the inductive reactance, which increases with the inductance.

**Hint**: Recall the relationship between resistance, inductance, and impedance in an AC circuit.

4. **Current Calculation**:
- According to Ohm's law for AC circuits, the current (I) can be expressed as:
\[ I = \frac{V}{Z} \]
where \(V\) is the voltage of the AC source. As the impedance \(Z\) increases due to the insertion of the iron rod, the current \(I\) will decrease.

**Hint**: Think about how increasing impedance affects the current in the circuit.

5. **Intensity of Light**:
- The intensity of the light emitted by the bulb is directly proportional to the current flowing through it. If the current decreases, the intensity of the light will also decrease.

**Hint**: Remember the relationship between current and intensity in a bulb.

6. **Conclusion**:
- Therefore, when the iron rod is inserted into the coil, the intensity of light from the bulb will decrease.

### Final Answer:
The intensity of light of the bulb will **decrease**.