The elements are onlyexcited to glow when energyis supplied. In 1905, Albert Einstein deduced from various theoretical considerations and experimental results that the energy of light is dependent on its wavelength and that this energy is only absorbed and emitted in defined "portions", so-called energy quanta. The light that the elements emit therefore carries exactly the amount of energy that they have previously absorbed. He was also able to derive a formula that describes the relationship between wavelength and energy. He was awarded the Nobel Prize in Physics in 1921 for this work. You can see the formula here:

\(E= \frac{\left(h\cdot c\right)}{\lambda}\)

with the Planck constant h = 6.626 - ^10-34 J - s

the speed of light in a vacuum c = 299,792,458 \({m \over s}\)

and the wavelength λ in metres

Using this formula, we can now calculate the energy emitted by the respective lines of the hydrogen spectrum. For the 4 lines in the range between 400 and 700 nm, this results in the following amounts of energy

From the fact that the light of a wavelength can be assigned a precise amount of energy, we can conclude that the amount of energy that an element can absorb and emit again in the form of light of a certain wavelength is also clearly defined. The atoms of each element therefore have specific, fixed energy levels that they can absorb.

If hydrogen atoms emit red light with a wavelength of 656 nm, they have previously absorbed an energy level of 1.89 eV. If they emit blue light with a wavelength of 434 nm, they have previously absorbed 2.86 eV of energy. However, we always see all lines in an emission spectrum at the same time, which means that if an element is excited to emit light, all these possible transitions between all possible energy levels take place at the same time.