A light bulb and an hollow coil inductor are connected to an ac source through a key as shown in figure. The switch is closed and after sometime an iron rod is inserted into the interior of the inductor. What will be the effect on the glow of light bulbs?

decreases

The correct answer is Option (1) → decreases

As the iron rod is inserted, the magnetic field inside the coil magnetizes the iron which ultimately increases the magnetic field having effectively increasing inductance of the coil. Therefore larger voltage will be consumed by inductor, leaving less voltage for bulb. Therefore, the glow of bulb decreases.