Hunting for spectra

Light is a type of radiation which allows us to see what is around us. The bodies from which light comes are defined “light sources” and can be of two kinds: natural and artificial.

Light seems to be white (or transparent) but is actually composed of all the colours of the rainbow, mixed together. The human eye perceives objects which do not emit light as coloured, only when the light (either natural or artificial) strikes them, is partly reflected and enters our eyes. For example, you see an object as red because when the light strikes it, it is all absorbed apart from the colour red, which is reflected, then enters your eyes and appears as such. This happens for all the other colours, apart from black and white, which - in fact - are not real colours, but rather the absence or sum of all colours, respectively. For a black object, all the light falling upon it is completely absorbed, and for a white one, all colours are reflected.

Light can be represented as a wave and in the image you can see multiple waves that represent different colours. The distance between two crests of the wave (or between two starts of the wave shape) is called wavelength, and is a fundamental feature of light. Each colour has its own wavelength, from the shortest (violet) to the longest (red). The unit of measure beside the number is the nanometer, which corresponds to a length a billion times shorter than a metre.

Passing from violet to red, the wavelength increases, and in the same space fewer waves crests are “included”, because a complete cycle takes up more space. An analogy could be made with drops of water falling into a glass: each time a wave “peak” arrives, we could imagine that it would transfer a “fixed” amount of energy… as if it were a drop of water falling into a glass. In a given time, the blue light carries more drops than the red light; namely, the blue light carries more energy than the red light.

Usually, colours within the light are all mixed and cannot be distinguished. In order to arrange the colours of light we need something to separate them. It might be a tiny drop of water, a piece of crystal (you know grandma chandeliers with crystal drops?) or a “piece of glass” called prism.

The prism is a transparent object. The (white) light beam enters and is “refracted” (the physical phenomenon which takes place is called “refraction”): when it gets into the prism, each colour in the lightbeam undergoes a deviation, which depends on the wavelength of the colour itself. Red waves are diverted less than the violet ones. The same happens when the rays of light come out of the prism. In this way, the light getting out is no longer white, but rather a band of ordered colours, from red to violet: the rainbow, which scientists define as “spectrum”.

The spectroscope you will build in this activity will work exactly as a prism, breaking down the light in different colours. Thanks to it, you will see with your own eyes that different light sources may have different kinds of spectra. If you observe sunlight (BE CAREFUL: NEVER LOOK DIRECTLY INTO THE SUN, NOT EVEN WITH THE SPECTROSCOPE!), you will see a rainbow, while energy-saving light bulbs do not produce a complete rainbow, but only a few coloured lines against a black background. They are called linear spectra as opposed to the rainbow that you will observe from the Sun, which is defined as a continuous spectrum.

In the two cases above, the spectra have different forms because light is emitted by different physical processes. In the case of the Sun (or a "normal" light bulb), the visible light is produced because the source is very hot (thousands of degrees). In the case of the energy-saving light bulb, there is a (cold) gas inside the bulb whose electrons, which normally orbit at a certain distance from the nuclei, are excited and jump into more distant orbits, and then fall back after a while upon the original orbit. While doing this, they emit light. However, the light they emit depends on the difference of energy between the two orbits, and has therefore a precise colour, which represents the energy of that jump.

Each gas, from any chemical element, has its own electrons, more or less far from the nucleus, and can make only certain jumps. Therefore the spectrum of a gas (i.e. the coloured lines emitted by the electrons of that gas, during their jumping) is typical of that gas. This is only its behaviour, and has that combination of colored lines (spectrum). In this sense, we can say that the spectrum is the fingerprint of the gas, of that element. If we see certain lines, we can deduce which chemical, or combination of elements, emitted them. You will therefore understand why it is important to watch celestial objects with a spectroscope: indeed, it allows us to learn what they are made of. In fact, in astronomy, this instrument is much more powerful: not only does it enable us to understand which elements constitute the source, we can also derive its temperature, the gas density, and whether it is approaching or moving away from us.