Why do light disappear?

[Keba]

I was wondering, why do light disappear?

I am new to physics. I do not understand why I cannot just turn my light on for a second, and the light will bounce back and forth for eternity. Some energy must be lost somehow, is that by colliding with solid objects or what? Also, what is light when it has lost its energy?

[dst]

Because of the form of light. Light, is just a stream of packets of energy. It doesn't disappear by itself but when it collides with atoms, the electrons absorb the light, and there are all sorts of effects, hence why you can even look at things and see them as solid, also why things have colours. So it would bounce back and forth for eternity assuming you could get perfect mirrors in a perfect vacuum. Energy loss, simple as that.

[Keba]

So by using mirrors, as in fiberoptics etc you can make the light last longer as it does not lose so much energy?
And what is the energy converted to under collision, heat or sound?

[mda]

I would say heat. However, note that heat is the vibration of atoms, so technically this reradiates both sound (due to mechanical vibrations) and also light (think of a heated object glowing).

[Keba]

Okay ^^ I think I understand a little more now.
Thanks for the enlightenment guys =)

[dst]

My understanding is sketchy, but different wavelengths of EM correspond to very different movements in the molecules - some wavelengths create wobbling/vibration, others translation, and high energy waves will rip electrons straight off.