SPECTROSCOPY FOR THE SCHOOL

Simón García
I.E.S. Aljada. Murcia (Spain)

Abstract

The spectroscopy is one of the branches of science that better information has given us on the structure and composition of the stars, nebulae, galaxies and of the Universe in general. Nevertheless it are accustomed to be absentee of the classrooms perhaps because it demand instruments normally expensive and sophisticated.

This Workshop tries to provide the needed material that allows to build a simple spectroscope and of cost economic zero, using a big box of matches and a piece of CD.

In this workshop we are also going to build another spectroscope, and spectrometer more elaborated that will allow us to carry out typical measures of laboratory easily comparable to astronomy.

We, in the workshop, will offer and they will explain didactic materials with practices directed to our students.

What is Spectroscopy?

Spectroscopy pertains to the dispersion of an object's light into its component colours (i.e. energies). By performing this dissection and analysis of an object's light, astronomers can infer the physical properties of that object (such as temperature, mass, luminosity and composition). But before we hurtle headlong into the wild and woolly field of spectroscopy, we need to try to answer some seemingly simple questions, such as what is light? And how does it behave? These questions may seem simple to you, but they have presented some of the most difficult conceptual challenges in the long history of physics. It has only been in this century, with the creation of quantum mechanics that we have gained a quantitative understanding of how light and atoms work.

What is a Spectrum?

How is it that we know so much about the chemical compositions, temperatures, pressures and motions of stars and galaxies which are so very distant that we would never dream of trying to travel to them? In order to answer this question we have first to ask how we know that these bodies exist at all. Well, quite simply, we know they exist because we can see them; that is, they are emitting energy in the form of waves light and also infrared, ultraviolet and often radio waves and X--rays as well. This energy travels over those vast distances and provides us with an extremely rich source of information about their make--up. A spectrum is the result of splitting up this light into its constituent colours and it is by studying spectra that astrophysicists have been able to make their most important discoveries.

The most familiar spectrum in nature is that splendid spectacle, the rainbow, which is produced when light from the Sun bounces around inside each of millions of raindrops and gets sorted out into its constituent colours in the process. When a chemist, physicist or astronomer wishes to examine a source of light he may use a triangular glass prism, or more commonly nowadays a device called a diffraction grating, to disperse the light into a spectrum. The whole apparatus for doing this job is called a spectroscope (if you look through it), or a spectrograph (if the spectrum is recorded photographically or by some means other than the eye). All modern spectrographs use diffraction gratings; the end result however, is rather similar to that produced by a prism whose action may be more familiar.

What Does a Spectrum Tell Us?

Isaac Newton in about 1666, while he was engaged in those experiments which were to lead to his construction of the first reflecting telescope, was the first to realize that the colours produced when white light is passed through a prism are a property of the light itself, rather than something introduced by the glass. This realization was to have extremely far--reaching consequences for the whole of physics and for our understanding of the Universe in particular. The great revolution in physics which took place during the first few decades of this century led to a thorough understanding of the way in which atoms and molecules can absorb and emit light and other radiations. It was known long before that different chemical elements emitted their own characteristic coloured radiations or lines when heated in their gaseous state, but it was the understanding of the relationship between those lines and the structure of the atom or molecule that proved to be so important for the development of astrophysics.

A familiar characteristic radiation from a common element is the yellow--orange light emitted by sodium vapour. Almost all of the light from a sodium vapour street lamp comes out in two very close lines in the yellow--orange part of the spectrum; this same element is also responsible for the yellow colour produced when, for example, salted water (common salt is sodium chloride) used in cooking is allowed to boil over into the flame of a gas burner.

If we look at an astronomical spectrum, and see the lines characteristic of a particular element, then we can immediately say that element is present either in the star or galaxy itself or, in some special cases, in the space between a star and our telescope. This is important enough in itself but, so powerful are the techniques of spectroscopy, we can do much more than just detect the presence of a chemical element or molecule. The obvious next question to ask is: how much of each element is present in a particular star? In fact this is not a very easy question to answer but it can be done, and indeed has been done for several hundreds of the brightest stars in the sky and for quite a number of other astronomical objects besides.

Classification of spectrum

When a narrow beam of white Light is passed through a transparent prism it emerge as a band of colours which change from red at one end to violet at the other through the colours of the rainbow. This band of colour is called a continuous spectrum.
We get much the same sort of spectrum from the prism if the light entering it comes from a hot, glowing body, irrespective of what the hot body is made. If, however, the hot body is heated until it vaporizes and the light from the hot vapour is passed trough the prism the band of colours is weakened but is crossed by a series of bright lines. Each element in the vapour provides one or more of the bright lines, each in a definite position relative to the others. The same is true of glowing rarefied gas. Such a spectrum is known as an emission spectrum.
If the light from a glowing gas or vapour passes though a similar gas at a lower temperature before entering the prism the bright lines are replaced by dark lines. In this case we obtain an absorption spectrum.

The latter is just the situation which exists in stars since the outer layers of the atmosphere are at a low pressure and cooler than the inner layers. Thus we can expect the continuous spectrum of a star to be crossed by a number of dark lines, although some of the hottest stars exhibit emission lines as well.
The stars can be classified according to the spectra which they produce. About 95 per cent of the stars can be put into classes which are labelled O, B, A, F, G, K and M, according to special characteristics of the spectra. For example, in type A stars the lines representing hydrogen are prominent, whereas in type G stars the lines representing calcium are strong and the hydrogen lines are considerably weaker.
The order of the classes given above is also the order of the surface temperature of the stars, the ones of type O having the highest temperature. It is also generally the order of absolute magnitude of the star.

The diffraction grating

Light from the star comes to us in a succession of waves, as in Figure.1. The wavelength of the light is also defined in Figure 1.

If we have two slits emitting monochromatic light, that is light of one wavelength only, such that the narrow slits A and B are the same distance from an observer O, then the crests and troughs from A and B will reach O at the same time, and we say that the waves are "in phase". In this case the illumination at O will be bright due to reinforcement of one wave by the other as in Figure 2.

If the distance which the light has to travel from B to a second point P is greater by half the wavelength of the light (l/2) than the distance from A to P, we shall get from A coinciding with crest from B. The waves are completely "out of phase" and darkness results at P. Similarly, if the distances travelled to a point Q differ by l, then we shall once more be in phase and brightness ensues. Thus we get a series of dark and bright lines on the line OPQ (Figure3).

The distance OP and PQ will, of course, depend on l, the wavelength of the light used. Thus for red light Q will be further from O than for violet light because lred is greater than lviolet. A white light should thus produce a spectrum in the general region Q. Due to the low number of sources this would be very faint, and we can intensify the spectrum by using many more slits very close together, so that the effect from each slit can add. Such a large number of slits may be obtained from a diffraction grating, on which can be ruled about 600 lines per millimetre, (Figure 4).

The size of the slits on the diffraction grating has to be about the same order as the wavelength of the light used, so that the whole of the diffraction grating is very compact indeed. The formation of several spectra from one diffraction grating can be seen in the next section, and indeed the diffraction grating can profitably replace the prism mentioned in Section spectrum.

Formation of multiple spectra

Figure 5 shows how first order and other orders of spectra are formed by rays of light whose paths differ by l, 2l, 3l and so on. It should be appreciated that the wave length of light is of the order of 6 x 10-7 metres whereas the distance AO might be of the order of 0.2 metres so that the diagrams are, of necessity, well out of scale.

A simple spectroscope

In this experiment we are you going to show how we can build a very simple and economic spectroscope, but that has an unequalable relationship quality/ price (measure for the power separator of the colours). Their separator power is based on the phenomenon of diffraction, produced in this case by microscopic "mirrors" for the reading of the laser in a compact-disc (CD). In a CD there is 1000 points of diffraction for each millimetre of disk, that it allows to separate the elementary colours very well

Material that you are going to need:

How will we build the spectroscope?

Put the piece of CD in the centre of the interior locker of the box of wax match. From such a form that upon opening a crack in the end of the box the reflected light and diffracted on the mirror incise in the window.

How could we use the spectroscope?

A more elaborated spectroscope

We will try to mount the spectroscope that J.Waxman (1984) described in. Astronomy. Cambridge University Oress, pp 320-323.
We can see the general aspect and the model of the window and wavelength graduation : Figures 10-11



Exploring Spectra using a spectrometer

The spectroscope can be used to look at the spectra of many different sources.

Incandescent light.
An incandescent light has a continuous spectrum with all visible colours present. There are no bright lines and no dark lines in the spectrum. This is one of the most important spectra, a blackbody spectrum emitted by a hot object. The blackbody spectrum is a function of temperature; cooler objects emit redder light, hotter objects white or even bluish light.

Fluorescent light
The spectrum of a fluorescent light has bright lines and a continuous spectrum. The bright lines come from mercury gas inside the tube while the continuous spectrum comes from the phosphor coating lining the interior of the tube.

Neon light
The simplest source of a neon light is a night light which says 1/4 watt on the package. These night lights have neon lights inside them. You can also find neon lights in the windows of businesses.

Warning: even though they are called neon lights the lights do not necessarily contain neon gas, some contain argon or other gasses to produce different colours. The red ones contain neon.

The spectrum of the neon light has several bright lines. The red lines are bright. The line used by helium neon lasers, 632.8 nm wavelength, does not appear in the spectrum of a neon tube. It is too dim relative to the other lines.

The lines of light are produced when electrons in an excited state decays into a lower energy state. The change in energy of the electron between these two states is precise and results in the emission of light with a narrow range of energies, a spectral line. DO NOT LOOK AT THE SUN! Even with a spectrometer.

Sunlight
Look at sunlight by looking at a white surface in the sun. White paper works well.

The solar spectrum is a continuous spectrum of an incandescent gas.
Look closely and you will see fine dark lines crossing the solar spectrum.
These fine lines are Fraunhofer lines. The dark lines are produced by gas above the surface of the sun which absorbs some of the incandescent light from the sun below. Each of these lines is produced by one atom or ion. However several lines may be produced by one atom. Two lines close together in the yellow are a famous pair of sodium lines.

Light emitting diodes, LEDs
These come in many colours from red, orange, yellow and green to blue.
In Light emitting diodes electrons in a higher energy conduction band drop into holes in a lower energy band. The energy lost by the electrons is emitted as light. Thus there is usually one brightest colour of light that appears as a line in the spectrum of the LED. In addition to the bright line there is usually also a dimmer, continuous emission of lower energy light. This lower energy light is produced when electrons decay to or from impurity states between the main energy bands.
In a solid the well defined energy states of electrons that would appear in atoms of a gas are spread into energy bands.

Street lights

Computer Screen

Look at a white screen on a computer. Notice the bright spectral emission bands.
Compare the spectral bands on a liquid crystal display screen to those on a cathode ray tube display.

You can also look at: Candles and Aurora

Diffraction Grating


You can also look at lights through a diffraction grating without using a spectrometer. Just hold the grating in front of your eyes and look through it at a light.

This only works for lights which appear to be small points of light or narrow lines of light that line up with the lines in the diffraction grating.
The diffraction grating spreads the light right and left when its lines are vertical. So look at a vertical line of light with the diffraction grating lines also vertical, i.e. the spectrum to the right and left. Look at horizontal lines with the diffraction grating horizontal, i.e. with the spectra above and below the light.
I usually place the diffraction grating in a plastic page protector to protect it from scratches and fingerprints.

Candle
A candle across the room works well. You will see the continuous spectrum of the incandescent carbon particles in the flame.

A linear filament incandescent light bulb or a distant light bulb The continuous incandescent blackbody spectrum will appear.

Stars
Few stars are bright enough to trigger the colour sensitive cones of your eyes. However those that are such as Sirius in the winter and Vega in the summer will have a continuous incandescent spectrum. If you look at stars through a telescope you will gather more light and be able to see their colours better. Hold the diffraction grating in front of a small telescope or behind the eyepiece of a large one.

Lightning
Lightning usually makes bright vertical lines. So hold the lines of the diffraction grating vertical to spread the spectra to the sides. Look at lightning and you will see the continuous spectrum from hot incandescent gas plus spectral lines from excited atmospheric gasses.

Light emitting diodes, LEDs
These come in many colours from red, orange, yellow and green to blue. These can be viewed at a large enough distance that they are small. You will see bright narrow band of light plus a broader dimmer band.

Sunlight
To see the solar spectrum never look at the sun.
Make a large black region using black paper or cloth.
Put a bright white line down this blackness. Look at the line through the diffraction grating. You will see a continuous spectrum. It is difficult to see the Fraunhofer lines.

Lasers
Never shine a laser beam into your eye!
However, you can project a laser dot on a wall and look at the dot through a diffraction grating. You will see just one dot of light spread to either side of the original dot representing the single colour of light produced by the laser.
You can also shine the laser through the diffraction grating at a distant white screen or wall. Once again a single dot of light will be diffracted to each side. Each single dot represents the single colour produced by the laser.

Other
Some sources that will not work well with a diffraction grating.
Aurora are too broad and diffuses to produce a good spectrum through a diffraction grating. Fluorescents in fixtures are too wide also.

References