Properties of Photons

Light travels as a wave but delivers its energy in discrete packets called photons. Every photon's energy is tied to its wavelength through two linked equations: c = lambda nu, where c is the speed of light, and E = h nu, where h is Planck's constant. Combine them and you get E = hc/lambda. Short wavelength means high frequency means high energy; long wavelength means low frequency means low energy. This is why a hydrogen lamp glows in only a few sharp colors instead of a full rainbow. Electrons in a hydrogen atom can only sit at certain fixed energy levels, so when one drops to a lower level, it emits a photon with exactly the energy of that gap, no more, no less. Only a few gaps exist, so only a few wavelengths of light come out, each one a sharp line in hydrogen's spectrum.

This idea sets up everything that follows about atomic structure. The energy gaps between levels in an atom are fixed, so the photons absorbed or emitted are fixed too, which is why each element has its own unique spectral fingerprint. It also connects back to solutions and concentration: when light shines through a colored solution, the wavelengths it absorbs depend on the energy gaps in the molecules present, which is the basis for reading concentration with light later in the unit. Photon energy is the bridge between the wave picture of light and the particle-level structure of matter.