Bohr energy ladder

A FIGURE 9.10 The Bohr energy ladder Bohr orbits are like steps on a ladder. It is possible to stand on one step or another, but impossible to stand between steps. 

Movement up the ladder of energy levels (an upward transition) is only possible if the molecule or atom absorbs a photon whose energy exactly equcds the energy gap (A ) involved in the jump. (This requirement is called the Bohr condition.) This process (termed absorption) is shown in Fig. 20.3. During absorption, the photon vanishes. The relationship between the energy jump and the frequency of the absorbed light is... 

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