Light is grainy
For most of the 19th century light was treated as a continuous wave — and for most everyday optics that picture works perfectly. The trouble is that nature occasionally hands you experiments (the photoelectric effect, the photon-counting statistics of dim sources) that only make sense if the wave is also a stream of indivisible packets called photons.
Each packet carries a fixed amount of energy that depends only on the color of the light. The relation is famously simple:
E = h · ν = h · c / λwhere h is Planck's constant, ν the wave frequency and λ the wavelength. Plug in numbers and you get a handy thumb rule for everyday spectroscopy:
E (eV) ≈ 1239.8 / λ (nm)Try it
Drag the slider below and watch how energy and wavelength stay locked together. The colored band shows roughly what the human eye sees; anything past the right edge is infrared, anything past the left edge is ultraviolet.
Conversion formula: E (eV) ≈ 1239.84 / λ (nm). The simulator's default probe sits at 1.3696 eV ≈ 905 nm, firmly in the near-infrared — invisible to the eye but very visible to a photodiode.
Why this matters here
Throughout the rest of the wiki we'll talk about photon energies in eV. Every reflectance plot in the simulator has eV on its x-axis. Every exciton has its own preferred energy. Every Bragg mirror is tuned to a target wavelength. They are all the same idea looked at from different ends.
- Light comes in packets whose energy is set by their wavelength.
1239.8 nm·eVis the conversion you'll keep bumping into.- Our experiment lives in the near-infrared (~900 nm ≈ 1.37 eV).