Eli5: why do reflection and refraction happen?

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Eli5: why do reflection and refraction happen?

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The electrons in a material move in response to the electric and magnetic fields that are light. This movement will create more waves – more light – which combines with what’s already there.

Inside of the material, the constant creation and combination of new waves causes the wave to seem slower. The new and old waves, when added up, create a wave that’s slightly behind the old wave.

At the very surface of the material, part of the new wave escapes as a reflection, and part of it combines differently with the old wave to weaken it slightly.

The result is that light appears to move slower inside of a material, and some of it reflects at the surface. All of the ‘new’ light is really just repurposing energy from the ‘old’ light and so at the end of it all the amount of light does not increase – that would be impossible.