If the speed of light is a constant @ 186000MPS. does this mean that it goes on for ever?

Question

If it can't slow down, what happens to it. If I shine a torch between two mirrors why does it disapear when the light is switched off. I have tried to imagine a photon like a bullet but with endless velosity but I know that it can't work, I am only capable of thinking in practicable terms, so forgive me if it is a stupid question

Answer 1

Anything moving at any velocity will go on forever unless something stops it. That is Galileo's law and Newton's first law. Light can be stopped by an opaque object. It can be slowed down by passing into a transparent object of high refractive index. The speed of 186000 mps applies to a vacuum only.

In theory, a collimated light beam will reflect between two (perfect) mirrors forever. In practice, it is impossible to perfectly collimate a beam. so the beam will grow in width and become fainter over time. In addition, it is difficult to eliminate all opaque or absorbing objects between the mirrors; even only one molecule of material will absorb some of the light at each reflection until the light disappears.

EDIT: I forgot what is probably the most important reason that light reflected between two mirrors eventually disappears. I have added the term "perfect" regarding the mirrors: that means they reflect 100% of the light. In practice, this is impossible to achieve, and at each reflection some light will be lost. If the mirrors reflect 99.99% of the incident light, and they are 1 m apart, the light intensity will be down to 1/1000 th of its initial value after only 0.0009 seconds.

Answer 2

I you look at the stars you are looking at light that has been traveling for hundreds to thousands of years.

the mirror idea wont work because you would need to shine the light at 90 deg to the mirror, and the torch would be in the way.. as the mirror surface is not perfect the light will bounce all over the pace etc.

I did see one test where they had a set of mirrors aligned up in a loop over several miles and they could measure the time it took for the light set off and return to the start point.

Answer 3

The energy goes on forever. Light is energy and if it interacts with some matter, it can be changed to heat but the energy is still there. Energy cannot be created or destroyed - just it's form changes.

Answer 4

Basically while the speed is constant it is absorbed and scattered by the air and everything else it hits.