Atoms and light is an interactive 3D simulation of atomic structure and the light atoms emit and absorb. You build any atom from its protons, neutrons and electrons, then use Bohr's model to see why each element only ever deals in its own fixed set of colours: a photon is absorbed or emitted exactly when its energy matches the gap between two energy levels (E_photon = E_m − E_n). It covers nuclide notation, energy levels, the photon relation E = hf, emission and absorption spectra, and the wave-particle duality shown by the double-slit experiment — the light-and-atoms topic in Danish upper-secondary physics.
The Bohr model says an atom's electrons can only sit on fixed energy levels (shells), never in between, and that a photon is absorbed or emitted exactly when its energy equals the gap between two levels (E_photon = E_m − E_n). For hydrogen the levels follow E_n = −2.18 aJ/n², which is why each element produces its own characteristic set of spectral lines.
A photon's energy is E = hf, where h ≈ 6.626 × 10⁻³⁴ J·s is Planck's constant and f is the frequency. Using c = fλ this can also be written E = hc/λ, so a higher frequency (shorter wavelength) means a more energetic, bluer photon.
An emission spectrum is bright lines on a dark background, produced by an excited (hot, thin) gas emitting photons at its own energy gaps; an absorption spectrum is dark lines cut into a continuous rainbow, produced when a cool gas removes those same wavelengths from white light passing through it. The bright emission lines and the dark absorption lines of one element sit at exactly the same wavelengths — one fingerprint seen two ways.
The Balmer lines are hydrogen's visible spectral lines, produced by electrons falling down to level n = 2: Hα 656 nm (red), Hβ 486 nm (blue-green) and Hγ 434 nm (violet). They are the only hydrogen series in visible light — jumps down to n = 1 (Lyman) are ultraviolet, and jumps to n = 3 (Paschen) or n = 4 (Brackett) are infrared.
Once a which-slit detector records the path, the two routes can no longer interfere, so the striped pattern collapses into two plain bands behind the slits. Without measurement each single particle behaves like a wave passing through both slits at once and builds up interference; knowing the path forces particle-like behaviour — the core of wave-particle duality and the quantum measurement problem.