What Exactly Is Light.
Light is a form of energy.
It is defined as the electromagnetic radiation with wavelengths between 380 and 750 nm which is visible to the human eye. Light has properties of both a particle and an electromagnetic wave.
Electromagnetic spectrum consist different type of wavelength and frequencies.
Gamma Ray's, X-rays, Ultraviolet, Visible light, Infrared, microwave,radio......
Source:- Kurzgesagt- In a Nutshell
What happens when a light bulb is turned on.
When a bulb is turned on, it emits millions of photons(a fundamental particle of visible light) which is scattered in all directions. This photons that is emitted hits every object that comes in it's path. This objects absorbs majority of the photons striking them also reflecting a fraction. This light is then reflected back to our eyes which helps us to see objects.
What happens when a light bulb is turned off.
As long as the bulb is turned on the room will have a constant supply of photons hence illuminating objects in a room. However, the moment the bulb is turned off things changes rapidly. As no new photons is emitted so there is no new photons to be absorbed and reflected by objects in a certain room. Which means complete darkness.
Darkness is just the absence of photons, so don't be afraid of it.
[Reference](https://www.scienceabc.com/eyeopeners/where-does-the-light-go-when-you-switch-off-a-lamp-inside-a-room.html?isamp=1)