Interesting and fun, but about 35 seconds in he says that you can’t combine photons together, and that’s just plain wrong.

I know that you can’t mix photons together. So you can’t take a blue photon and a green photon and mix them together to get some other photon. That just doesn’t happen.

Except that it does. You have to do this in a nonlinear medium like certain crystals, but it can be done. It’s called sum frequency generation.

Energy is conserved, so in the example given a 2.5 eV (green) photon added to a 3 eV (blue) photon will result in a 5.5 eV photon, which will be ultraviolet. The addition is not what we see with our eyes, since that’s a different process.

A special case of this is where the two photons being added are the same frequency. This is called frequency doubling, and a common (amongst geeks, at least) example of this is a green laser pointer. The source inside of this is actually an infrared laser emitting at 1064 nm, which then passes through a doubling crystal to produce light at twice the frequency, or half the wavelength: 532 nm. (and cheap laser pointers may not filter the IR from the output, which can be a danger)