Energy can't be destroyed?

[JackWild]

If energy can't be destroyed only transformed into a nother form of energy then can someone answer the following?

Put a torch in a mirrored light proof box where no light can escape make it remotely turn its self on and then off.

No light can escape from the box yet the light disappears as soon as the torch switches its self off.

Yes, I am aware that the bulb will also produce heat (in fact probably 90 percent of a regular bulb's energy is used making heat not light) but I am allowing for this and would like to know what happens to the light portion.

[fo3nix]

If no light can escape from the box, how do you know it turns itself off? (or indeed on again)

How do you send a signal to it if no light can get in or out? Any signal would probably be electromagnetic, thus light at a different frequency.

[JackWild]

Ok then the light works on a timer. No remote control required.

[mrbatfink]

Light is made up of photons traveling at the speed of light (funny enough)..... Once these photons slow down they are no longer light (for it is not light if it does not travel at the speed of light).

Anyhow the mass of a photon is so small you can pretty much discount it... So the answer you are looking for is motion. The photons slow down by bumping in to things causing them to move. Therefore no light but motion.

I am no physics professor but that is how i understand it.

[GuavaHalf]

A couple of thing:

1) No mirror is perfect. Even if the mirror reflected 99.9% of the light hitting, all of the light would be absorbed very quickly - when I say absorbed, what I mean is that the mirror would heat up very, very slightly.

2) If the light bounced off the mirro, some of it would bounce back to towards the torch, which is not a mirror and wich would therefor absorb the light.

[GuavaHalf]

Please forgive all of the typos in the last answer!!

[CT]

No material is 100% reflective. Therefore some thing must happen to the photons that hit any surface. They are either reflected or absorbed. In your sealed box, those that are reflected will just bounce off in other direction until they meet another surface. Those that are 'absorbed', hit one of the electrons around an atom and increase its energy. This in turn can pass on energy through conduction within the material. The heat from your light bulb is just another type of EM radiation, like light. Only it cant be seen. This escapes the same way.

[cliffsdoll]

dont know, but loved you in oliver and flight of the doves

[jake-the-peg]

It depends on whether you are thinking about an idealised "perfect" system. In this case the light would continue to be reflected back and forth. This is the basis of the "light clock" used in the famous time dilation thought experiment in special relativity.

As has already been said, if it's not a perfect system the energy will rapidly be absorped by the mirrors.

regarding mrbatfink's statement about slow photons - when light travels through something it does slow down. In glass it's typically 2/3 of it's speed in a vacuum the ratio of the speeds at a material boundry gives us the refractive index - (Snells law)

[xrayspecs]

The light disappears because you turn it off.... what's difficult about that? The light only existed in the first place because the torch was ON. You don't need a light proof box to see this happen shine a torch in your face then turn it off - Light ON - Light OFF.. Simple really!

[mrbatfink]

I stand corrected...

if two light photons have an exact head on collision do they stop and cease to by light? surely it take some energy to change the direction if light? Am i correct in thinking photons do have some mass no matter how small? if so surely the light would stop eventually?

i am sure i am probably wrong again but i need to know.

[CT]

Photons are defined as massless particles, hence their ability to travel at the speed of light. In a collision between two 'light' energy photons they would just bounce of each other. Higher energy photons, such as gamma rays, can collide and become two electrons.

[mrbatfink]

so E=MC2 does not apply to light?

[CT]

E=mc^2 applies in certain situations. Photons have no mass, but if they are involved in a collision where they change to a particle with mass then it can apply. As I said earlier, to Gamma ray photons can collide and become two electrons, or other combinations of subatomic particles. In this case the energy is converted to mass during the collision. The energy initially, divided by the square of the speed of light is equal to the new mass.