© Springer Nature Switzerland AG 2021
B. G. Sidharth et al. (eds.)Fundamental Physics and Physics Education Researchhttps://doi.org/10.1007/978-3-030-52923-9_21

21. Design, Construction and Use of a Quantitative Spectroscope for Science Dissemination

Francisco Savall-Alemany1 , Jesús Carnicer Murillo2, Rocío Espinosa Gutierrez3, Esther Perales Romero4 and Hernán A. Supino Graña2
(1)
Universidad de Alicante, Alicante, Spain
(2)
Museo Didáctico e Interactivo de Ciencias de la Vega Baja del Segura de la Comunitat Valenciana (MUDIC-VBS-CV), Alicante, Spain
(3)
Stowmarket High School, Stowmarket, UK
(4)
Grupo de Visión y Color, Departamento de Óptica, Farmacología y Anatomía, I. U. Física Aplicada a las Ciencias y la Tecnología (IUFACyT), Universidad de Alicante, Alicante, Spain
 

Abstract

In this work we present a spectroscope that we have constructed for science dissemination in museums. The materials used are economical and easy to handle, allowing all attendees in the spectroscopy workshop to build, use and take home a small but useful spectroscope. We also present the steps followed to construct it and the STEM activities that are carried out in the museum to learn how to use it.

Keywords
SpectroscopyScience museumScience dissemination

21.1 Introduction

Spectroscopy has been a technique of great scientific relevance, practically since its appearance. It contributed to the discovery of elements as caesium, rubidium or helium; it was used to determine the composition of the Sun and other celestial objects, to establish the structure of atoms, etc. Nowadays, its use is common in the detection of chemical substances or in the study of celestial objects, among many other applications. In addition, since the popularization of LEDs as a light source, several studies have been carried out which highlight their impact on the reconciliation of sleep, an effect that is related to the spectre of the light they emit (Gooley et al. 2011). Despite its scientific importance, our experience shows us that it is completely unknown to the general public. On the other hand, in the school and academic world, spectroscopy has an important presence due to its relationship with the atomic models studied in Physics and Chemistry at high school. However, didactic research has found that even university science students have difficulties to explain spectrum formation (Ivanjek et al. 2015; Savall et al. 2016).

At the Museo Didáctico e Interactivo de Ciencias de la Vega Baja del Segura de la Comunitat Valenciana (MUDIC-VBS-CV), we have set the objective of bringing spectroscopy closer to the general public through hands-on and STEM activities. Visitors are involved in activities in which they have to build a simple spectroscope and use it to observe and interpret the spectra of common light sources such as energy-saving bulbs, LEDs or old tungsten filament bulbs.

21.2 Project Description

Building a spectroscope is a simple task that most science professors have carried out and that has been presented in numerous publications (De Luís and Martínez 2010; Remón et al. 2016). However, the main part of the proposals is based on qualitative spectroscopes, being the quantitative spectroscopes difficult to construct due to the materials used. Our work has focused on that objective: the construction of a spectroscope with economical and easy-to-find materials that allows us to measure the wavelengths of spectral lines.

The Museo Didactico e Interactivo de Ciencias de la Vega Baja del Segura de la Comunitat Valenciana (MUDIC-VBS-CV) is mainly visited by schoolchildren (and when the accompanying teachers request it they carry out a spectroscopy workshop) as well as general public that visits the museum during open days or when offering other activities. In this workshop, visitors construct the spectroscope with the help of a museum monitor and use it to analyse the most common light sources: incandescent lamps, fluorescent lamps, LED lamps, etc.

The main aim of the workshop is that many families from the provinces of Alicante and Murcia build the spectroscope and use it to analyse the different light sources. By that, we will contribute to achieve the objectives of the MUDIC-VBS-CV, which refer to the dissemination of scientific content outside the school. Visiting teachers will also be able to borrow spectroscopes for their classrooms. The spectroscope will also be provided to teachers in the training courses organized by the MUDIC-VBS-CV, so that it can be used to deal with the curricular contents related to spectroscopy.

21.3 Methodology and Materials

For the construction of the spectroscope, we use an A4-size cardboard, light coloured or preferably white. On the cardboard we print the template of the spectroscope, which is provided to the attendants, together with a small diffraction grating (Fig. 21.1).
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Fig. 21.1

On the left it is shown the outer part of the spectroscope which includes a brief explanation of spectroscopy history. The right image shows the inner side, which includes an explanation of the mounting and use of this spectroscope. It is offered to the attendees printed and die-cutting. Its real dimensions are 18 cm × 8 cm × 4 cm

We have designed the template from other spectroscope models (see, for example, Savall et al. 2013). In this case, we have reduced the spectroscope length and we have calibrated the scale to be able to read the values of the wavelengths of the spectral lines directly, a useful improvement that allows us to read the wavelength directly avoiding the use of mathematical functions.

Once mounted, we obtain the spectroscope shown in Fig. 21.2. It consists of a box of 18 cm × 8 cm × 4 cm. In one of the sides it has a rectangular hole, the ocular, which is covered with a diffraction grid of 465 lines/mm. On the opposite side, aligned with the grid, it has a narrow slit that is the objective of the spectroscope. Inside the box, to the right of the slit, it has a scale that allows the user to measure the wavelength of the spectral lines in nanometres. To observe the spectrum of any light source, the diffraction grid and the slit must be aligned with the source. The spectrum is seen on the scale, which gives the wavelengths of the spectral lines.
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Fig. 21.2

On the left it is shown the spectroscope once mounted. At the bottom we can see the slit behind which the light source is located and to the right we appreciate the calibrated scale for measuring the wavelength. On the front, we see the ocular, covered by a diffraction grid. Next to the spectroscope there is a 2-cent euro coin which serves as a reference for appreciating its small size. On the right side of the image we show two spectra, corresponding to a white light emitting LED (top) and a low-energy light bulb (bottom)

To calibrate the spectroscope, we analyse the diffraction phenomenon produced by the grid. When a wavefront reaches a diffraction grid, each of the slits in the grid becomes a new source of new wavefronts. On each point of a screen located at a distance L from the diffraction grid (as shown in Fig. 21.3), there will be an overlap of these secondary wavefronts. At those points on the screen where the waves arrive in phase, constructive interference will occur and a maximum intensity will be detected. In fact, if we point a laser (monochromatic light) through the diffraction grid we observe a series of light points on the screen where the interference is constructive. If we change the colour of the laser, the light points are placed in different positions, because of the different laser’s wavelength.
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Fig. 21.3

Diffraction grid containing slits at a distance d. At a distance L from the grid, we set a screen on which the light spectrum is observed. We consider an X-point where constructive interference occurs. Red and green lines represent the paths followed by the waves from two consecutive slits to X-point and Δ represents the difference in both paths’ length

Considering the waves that reach point X on the screen (as shown in Fig. 21.3) from two consecutive slits in the diffraction grid, the paths’ length difference for both waves corresponds to the length of the segment Δ. We can then set the relationships:
$$ \sin \alpha =\frac{\varDelta }{d} $$
and
$$ \sin \alpha =\frac{x}{\sqrt{x^2+{L}^2}} $$
By combining both expressions we find that
$$ \frac{\varDelta }{d}=\frac{x}{\sqrt{x^2+{L}^2}} $$
that relates the paths’ length difference to the X-point of the screen.
The interference will be constructive where the path difference is a multiple of the wavelength:
$$ \varDelta = n\lambda $$
In our case, we will work only with n = 1, therefore:
$$ \frac{\lambda }{d}=\frac{x_{\mathrm{max}}}{\sqrt{x_{\mathrm{max}}^2+{L}^2}} $$
So the positions of the maximums will be:
$$ {x}_{\mathrm{max}}=\frac{\lambda L}{\sqrt{d^2-{\lambda}^2}} $$
If we use a non-monochromatic light source, by using this last expression we can determine the position in which we will observe the maximum of intensity for each wavelength, that is, for each spectral line. Then, we can draw a scale to directly measure the wavelength of the radiation. In our spectroscope, we have drawn a line on the right of the slit in the positions shown in Table 21.1 for the seven values of the wavelength.
Table 21.1

Position where each spectral line is observed

λ (nm)

400

450

500

550

600

650

700

x (cm)

3,4

3,9

4,3

4,8

5,2

5,7

6,1

When we use the energy-saving bulb as the light source, we obtain the spectrum shown in Fig. 21.2 and the scale allows us to measure the wavelength of each spectral line. In Table 21.2, we show the wavelengths measured using the spectroscope for each spectral line of the energy-saving bulb. Some references (Sansonetti et al. 1996; Kraftmakher 2010, 2012) allow us to obtain the wavelengths emitted by these bulbs. We observe in Table 21.2 that the measured and the tabulated wavelength are similar, within the limits that can be expected for an inaccurate instrument like this.
Table 21.2

Measured wavelengths for each spectral line of the low-energy bulb and values tabulated in the literature

Line

Violet

Blue

Green

Orange

Red

λmeasured (nm)

400–450

500

550

600

650

λtabulated (nm)

405 and 436

492

546

577

612

We have also tested the spectroscope with three laser pointers (red, green and blue) with wavelengths of 635, 532 and 405 nm, respectively. By measuring the wavelength with the spectroscope, we confirm that the wavelength measured by using the spectroscope matches the value provided by the laser devices, within the precision expected.

21.4 Teaching and Learning Sequence Developed in the Workshop

The teaching and learning sequence (TLS) is carried out with 20 attendees in two different rooms, each of which has a maximum of 10 attendees. Each group is guided by a museum monitor. The workshop is developed through an inquiry-based strategy.

At the very beginning, the TLS sets a problem and involves the attendees in a research aiming at finding an answer using their own previous knowledge and with the assistance of the monitor. In our case, the problem is “Which visible light sources do we currently have? What light do they emit?” After setting the problem, we carry out some activities to familiarize the attendees with the problem: we show the attendees some light sources (an image of a star, a flame, some different bulbs or a TV screen, among others) and we ask them to think what they have in common, whether they emit light of the same colour and what colour of light would be most useful for their purpose.

Once we have familiarized the attendees with the light sources and the colour of the light emitted by each one, we introduce the possibility of “breaking down” the light into spectra to analyse its chromatic composition. To do so, we show them an image of a natural rainbow and ask if a similar analysis of artificial light sources could be done using artificial instruments. After showing them a diffraction grid and stick it into a spectroscope that they will later mount, we use it to observe the light spectra of the different artificial sources previously introduced. We asked the audience about the similarities and differences between the observed spectra.

The assistants observe that there are light sources that have the same spectrum (the fluorescent light tube, the energy-saving bulb and the mercury Crookes tube) and associate it with the similarity between the bulbs. The monitor explains that sources with the same spectrum contain the same gas, in this case mercury. Then, we try to explain why a substance always emits the same light spectrum. We reflect on the nature of light and introduce the idea that it is a beam of photons and that changes in the energy of the atom produce these photons. Therefore, if the changes are limited (which implies that the energy in the atom is quantized) the light emitted will always be the same. By reflecting on this explanation, the assistant can explain (in a non-formal way) how the other sources emit light.

After completing this first part, the attendees move on to the other room where they will build a spectroscope (Fig. 21.2) with the help of the monitor. The spectroscope is already printed and die-cast on a cardboard sheet, so not any additional instruments to assemble it is required. While assembling it, attendees need to reflect on its structure and determine where the slit, scale and diffraction grid should be located. As attendees construct the spectroscope, they discuss the usefulness of the spectroscope in everyday life, its scientific usefulness and industrial applications. Once the spectroscope has been built, we encourage the attendees to use it to prove it by observing the spectrum of light sources in the room and to explain them qualitatively using the concepts of electronic states and electronic transition introduced in the first part of the workshop.

21.5 Conclusions

By using simple instruments and economic materials, we have been able to design and build a quantitative spectroscope that can be used both in scientific dissemination activities and in the classroom. In fact, the work carried out in the classroom with the spectroscope shows an increment of the students’ interest in this analysis method and a better learning of the academic contents. Likewise, its use in the museum has aroused the general public’s interest in light-emitting processes, their technological application and their social and economic consequences.