When a light bulb is lit, not all of the energy is transformed into light. Some of the energy is transformed into what?

Some of the energy is transformed into heat or thermal energy

Explanation:

Energy Conversion

When we turn a light bulb on, the electric energy is transformed into light, but some part of it is also converted into heat or thermal energy.

Incandescent light bulbs convert a great part of the electric energy into heat because it needs to heat the filament for it to emit the electromagnetic radiation and glow.

Other types of light bulbs are more efficient in terms of less heat loss in the process. The most energy-efficient light bulbs are the Compact Fluorescent Light Bulbs, and the Light Emitting Diode or LED bulbs.