Atomic Models – Atomic Spectra

Sir Isaac Newton split white light into spectral colours using a prism. We have built a similar experiment. The white light falls on the prism, which splits it up into its constituent colours forming a rainbow.

However, if we choose a specific gas as a light source – in this case it is neon – only certain spectral lines will appear. This is why the light emitted by a neon tube is not white, but yellowish-orange. For helium, we will obtain a different set of spectral lines. Hydrogen emits only four spectral lines in the visible range.

Joseph von Fraunhofer used a similar set-up to study the spectrum of sunlight. He discovered dark lines in the colour spectrum. Today, we call those lines Fraunhofer lines. Apparently, specific frequencies in the solar spectrum are absorbed before they reach the Earth.

Each element has a unique emission spectrum, similar to fingerprints. We can thus infer which elements are present on the solar surface, based on the black absorption lines. This is how the element helium was discovered on the Sun’s surface at the end of the 19th century. Nowadays, we can observe the sunlight in different wavelengths, or spectral ranges, directly from outer space.

Spectral lines are fingerprints and distinctive properties of elements, and can be measured as such. Following this discovery, we are faced with the obvious question: what causes these spectra? We shall discuss the key milestones, which have led to the current explanation offered by quantum physics, in the following slides.