Speed of light

[Bluenoser]

Bearing in mind that energy cannot be created or distroyed-only changed, can you tell me the answer to a question?
When light enters a medium, such as glass or water, it is deflected because it slows down while passing through it and loses energy (glass gets warm in sunlight). However when it exits it is back to its original speed. Where did the energy come from to accelerate it back to its full speed?

[Loosehead]

The speed of light is constant for a given density of material. It does not lose/gain energy it does not deccelerate or accelerate.

[Bluenoser]

I can't follow your reasoning. When sunlight passes through a window pane, its warms the pane, therefore losing energy, does it not?

[jake-the-peg]

Some of the light is absorbed this warms the media.

The light energy out equals the light energy in - the light energy absorbed.

An individual photon will either be transmitted or absorbed

[mibn2cweus]

Heating is due to absorption on some of the light energy, not all of the light passing through.

Refraction takes place at the interface between two mediums of differant density, not within a medium of constant density.

[Loosehead]

yes as mibn and jake say. Not all the light passes through the medium some is absorbed, the heat is not caused by deceleration.

[Bluenoser]

Thank you for your answers. I've wondered about this for some time, and you have certainly enlightened me.

[mibn2cweus]

Refraction takes place as photons of light entering a transparent medium are temporarily absorbed by the atoms in a transparent medium. The atoms quickly release the photons of light after absorption to return to a normal energy level. Once released the photons travel at the only speed they can, the velocity of light.

[laho pia]

just as a side issue, the heat that warms up the window pane is a form of light called infra red and it is not visible to the eye (just like ultra violet). The light that you see is not 'losing energy' is it refracting through the medium. I dont know if you've noticed but depending on the angle of 'incidence' the light beam can exit the medium wider than entering. To the point where it will seperate out into the 7 colours making up white light -a rainbow!

[dawkins]

When the light enters the denser medium the light waves contract. Light does not lose any energy as a result of passing through different mediums such as glass or water and does not gain energy on exiting the medium although it will revert back to c. The upshot of this is that it does not accelerate back up to full speed but rather has different speeds based on the medium involved.