Photon

Two equations on the AP Physics 2 sheet fix what a photon is: $E = hf$ and $\lambda = c/f$. Combine them and the energy is $E = hc/\lambda$, a form the sheet does not print but its constants block supports with $hc = 1240 \, \text{eV} \cdot \text{nm}$.

The distinction that costs marks is brightness against color. Intensity sets how many photons arrive per second and leaves each one's energy alone; frequency sets the energy of every photon in the beam. That is why a bright red beam can fail to free an electron that a dim violet beam frees, which is the whole content of the photoelectric effect.

Essential knowledge 15.1.A.2.i has the photon massless and electrically neutral, so no electric or magnetic field deflects it, and 15.1.A.2.ii has it traveling in straight lines unless it interacts with matter. It still carries momentum, $p = h/\lambda$, which is what Compton scattering exploits.

Speed is the other trap. Every photon moves at $c = 3.00 \times 10^8$ m/s in free space, but 15.1.A.3.ii makes the speed through a medium inversely proportional to that medium's index of refraction. Photons slow in glass.