# How does light change speed?

16 June 2015

## Question

We know that light is composed of photons that move at 300,000 km per second in a vacuum. If I shine a light though a 5cm sheet of glass, it will slow down and also heat up the glass as it passes through. As it leaves the glass, it then speeds up again, Now where does the energy come from to speed it back up to 300,000 per second?

One could assume that the continuous flow from the source achieves this, but what would happen if one directed a single photon through the glass sheet? Would it still lose energy to the glass as it passes through and then as it emerges, how could it find the energy (from nowhere) to speed back up to 300,000 km per second?

We put this question to Cambridge University physicist Zephyr Penoyre...

Zephyr - When it goes into the glass, it's moving slower. But also, because the frequency has to be the same, the same waves have to be coming into the glass at the same rate as they're coming out, otherwise you've lost waves somewhere along the way.

Chris - And it's changed colour.

Zephyr - And that's because wavelength has changed. So, the energy of the light is to do with the wavelength of the light. So, as it slows down, the energy gets higher - because the wavelength has shrunk - so the total amount of energy passing through the glass is exactly the same as it was going through the air, it's just moving at a slightly different speed.