The brightness of the bulb connected to a solenoid with an iron core will change when the core is removed. To understand this, let us break down the explanation into the following sections:

1. Introduction:
- The solenoid is an electrical coil typically made of copper wire wound around a cylindrical core.
- When a direct current (d.c) is passed through the solenoid, it creates a magnetic field around it.

2. Magnetic Field and Iron Core:
- The presence of an iron core within the solenoid significantly affects the strength of the magnetic field produced.
- Iron is a ferromagnetic material, which means it can be magnetized easily and has high magnetic permeability.
- When the d.c current flows through the solenoid, it induces a magnetic field in the iron core, increasing the overall magnetic field strength.
- The iron core acts as a magnetic amplifier, concentrating and enhancing the magnetic field lines passing through it.

3. Effect on Bulb Brightness:
- The brightness of the bulb connected to the solenoid is directly related to the strength of the magnetic field.
- When the iron core is present, the increased magnetic field lines passing through the solenoid result in more efficient conversion of electrical energy into magnetic energy.
- This enhanced magnetic field induces a stronger current in the bulb filament, causing it to emit more light and thus appear brighter.

4. Removal of Iron Core:
- When the iron core is removed from the solenoid, the magnetic field strength decreases significantly.
- Without the iron core, the magnetic field lines spread out more, reducing their concentration within the solenoid.
- As a result, the efficiency of energy conversion decreases, leading to a weaker current induced in the bulb filament.
- The reduced current results in the bulb emitting less light, causing it to appear dimmer compared to when the iron core was present.

In summary, the brightness of the bulb connected to a solenoid is directly influenced by the presence or absence of an iron core. The iron core amplifies the magnetic field, leading to greater energy conversion and a brighter bulb. Removing the iron core reduces the magnetic field strength, resulting in less efficient energy conversion and a dimmer bulb.