Answer:
- Because the atoms of the elements only emit some specific frequencies of light.
Step-by-step explanation:
The spectra lines are the result of the emission of different frequencies of light (electromagnetic radiation) from the atoms of an element when some electrons decay from an upper (excited) energy level to a lower energy level.
The fact that certain regions do not show lines means that the spectrum is not continuos but discrete. This is, only some specific frequencies of light are emitted.
The frequency of the light is related with the energy per the Einstein-Planck´s equation:
Where h is Planck's constant and f is the frequency of the electromagnetic wave (light).
Thus, it is concluded that only photons with certain specific energies are emitted.
This is what is meant when you say that the light has dual behavior: as wave and as particle: the light is emitted in packages named quanta.
The emission spectrum is a demostration of the discrete (quantized) nature of the light.