The atom that comes in steps. In the Bohr model of hydrogen an electron can only sit in fixed orbits, each with a definite energy En = −13.6/n² — never in between. Move it up to an excited level, then let it fall: it releases the energy as a single photon of wavelength λ = 1240/ΔE, and that light lands as a coloured line on the emission spectrum below. Drop electrons to level 2 and the famous red, cyan and violet Balmer lines of hydrogen appear one by one. It all runs on your device.
You are in the Physics lab.
What the orbits, the travelling photon and the two diagrams are telling you.
On the left, each ring is an allowed Bohr orbit and its radius follows the real relation rn = n²·a₀ — so level 2 sits four times farther out than level 1, level 3 nine times, and so on. The orange ring is your initial level ni, the indigo ring your final level nf, and the blue dot is the electron. Move the two sliders and watch which rings light up.
Press the action button and the electron slides from ni to nf. If it drops (nf below ni) it emits a photon that flies outward; if it climbs it absorbs one that flies inward. The photon is drawn as a travelling wave whose colour is set by the energy gap ΔE — true spectral colours in the visible range, and a muted tint labelled UV or IR when the line falls outside it.
On the right, the energy-level diagram stacks the levels at En = −13.6/n² — crowding toward 0 eV at the top — and draws the jump as an arrow (down for emission, up for absorption). Under it, a 380–740 nm strip shows where a visible line lands, or flags it as UV/IR. The lower spectrum keeps every line you make, so pressing Balmer series paints hydrogen's red, cyan and violet fingerprint at once.
Quantised orbits, a jump between them, and a photon whose colour is set by the gap.
Bohr's rule is that the electron can occupy only special orbits, numbered n = 1, 2, 3…, each with a fixed energy En = −13.6/n² electron-volts. Level 1 is the tightest and lowest; higher levels crowd together near zero. The electron can never sit between them, so the atom's energy is quantised — the energy-level diagram labels each level with its energy.
As the electron moves between nhigh and nlow it exchanges the energy difference ΔE = 13.6(1/nlow² − 1/nhigh²) as a single photon — emitting it on the way down, absorbing it on the way up. Because that energy is fixed, so are the colour and pitch: the wavelength is λ = 1240/ΔE nanometres and the frequency f = ΔE·e/h, the same relation as the Rydberg formula.
Every photon lands as a coloured line on the spectrum at its wavelength. Jumps to level 1 (the Lyman series) sit in the ultraviolet; jumps to level 2 (Balmer) fall in the visible as the red 656 nm, cyan 486 nm and violet lines; jumps to level 3 (Paschen) are infrared. Press Lyman, Balmer or Paschen series to emit a whole series at once and watch that fingerprint appear.
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