Answer:
From electrical energy to radiation
Step-by-step explanation:
A light bulb converts electrical energy to light, which is nothing more than radiation. Don't regard radiation as a bad thing, this light is non-iodizing radiation, which does not have the possibility to alter our DNA, and thus it's safe.
The simplest example can be found in old lights (obsolete now because they are vastly inefficient). This lights were just resistors which turned electrical energy into heat. A very hot material reflects light in the form of radiation, as described above.
Technologies like LEDs are more complicated to explain.