Question: If photons are just energy, why are they affected by objects?

Christina Pagel answered on 27 Jun 2013:

you certainly like the hard questions! 🙂

I had to brush up on my quantum mechanics to answer this which I’ve not done in a *long* time!

The easiest way to think about light is to think of it bouncing off stuff – and that’s fine on a large scale. But, you’re right photons are just energy with no mass… so what actually happens when a photon hits something (say your skin), is that its energy is imparted to an electron in the surface atom. This excited electron then jumps to a higher energy state and then decays again – and as it decays it emits a photon – that’s the photon that then hits your eyes (and makes it look like the original photon just bounced off). depending on the interaction between the photon and electron, photons will be emitted in different energy bands (that gives things their colour)…