More than meets the eye: the electromagnetic spectrum
Submitted by brown on 23 August 2011
We learn about the world around us via our senses. Our eyes play a major role, because light carries a great deal of information about its source and about the objects that either reflect or absorb it. Like most animals, humans have a visual system that collects luminous signals and relays them to the brain. Our eyes, however, are only sensitive to a very small portion of the spectrum of light – we are blind to anything but what we call ‘visible’ light.
Or are we? Over the course of the 19th century, scientists discovered and visualised several different types of previously invisible light: ultraviolet (UV) and infrared (IR) radiation, X-rays and gamma-rays, radio waves and microwaves. It soon became evident that visible light and these newly discovered forms of light were all manifestations of the same thing: electromagnetic (EM) radiation (see Figure 1).
The various types of EM radiation are distinguished by their energy: gamma-rays are the most energetic, followed by X-rays, UV, visible and IR light. Types of EM radiation with wavelengths longer than IR light are classed as radio waves. These are subdivided into sub-mm waves, microwaves and longer-wavelength radio waves. EM radiation propagates as waves that travel even in a vacuum. The energy (E) of the wave is related to its frequency (f): E = hf, where h is Planck’s constant, named after the German physicist Max Planck. The relationship between the frequency and wavelength (λ) of EM radiation is given by fλ = c, where c is the speed of light in a vacuum. These two relationships allow EM radiation to be described in terms not only of energy but also of frequency or wavelength.
Radiation at different energies (or frequencies, or wavelengths) is produced by different physical processes and can be detected in different ways – which is why, for example, UV light and radio waves have different applications in everyday life.
Towards the end of the 19th century, scientists began to investigate how this radiation from the cosmos could be captured to ‘see’ astronomical objects, such as stars and galaxies, in wavelengths beyond the visible range. First, however, they had to overcome the barrier of Earth’s atmosphere.
ESA is a member of EIROforumw5, the publisher of Science in School.
To see all ESA-related articles in Science in School, see: www.scienceinschool.org/esa
The opacity of the atmosphere is not the only challenge it poses for astronomers; its turbulence also impairs the quality of astronomical observations even at wavelengths that reach the ground, such as visible light. Faced with these problems, in the second half of the 20th century, following the birth of the space age, astronomers began to launch their telescopes beyond the atmosphere, into space. This started a revolution in astronomy comparable to the invention of the first telescope just over 400 years ago.
Figure 3: The Andromeda Galaxy, the nearest major galaxy to the Milky Way, seen at different wavelengths. Observations of visible light, with a ground-based telescope, show the several hundred billion stars that make up the galaxy. Observations at far-infrared wavelengths, by the Herschel space observatory, reveal the mixture of (mostly) gas and dust from which new stars will be born. X-ray observations, by the XMM-Newton space observatory, show the glow emitted by stars close to the end of their life cycle or by remnants of stars that have already died
Because different physical processes emit radiation at different wavelengths, cosmic sources shine brightly in one or more portions of the EM spectrum. By exploiting both ground- and space-based telescopes, therefore, astronomers today can combine observations from across the spectrum, which has produced a previously hidden and extremely captivating picture of the Universe (Figure 3 and Figure 4). Observations in the IR range, for instance, show the otherwise invisible mixture of dust and gas that fills interstellar spaces and from which new stars are born. By detecting gamma- and X-rays, astronomers can observe the most powerful phenomena in the Universe, such as black holes devouring matter and supernova explosions.
Figure 4: The Orion Nebula, an iconic cosmic ‘nursery’, seen at different wavelengths. The blue frame zooms in on part of the Orion constellation, and the orange frame zooms further in, showing the Orion Nebula in greater detail. This region, where thousands of stars are forming, looks very different across the EM spectrum. Observations in visible light, from ground-based observatories, show mostly stars, while observations at longer wavelengths (near- and mid-infrared, (sub)millimetre and microwave) reveal the intricate mixture of cold gas and dust from which stars are born. In contrast, X-ray observations show the extremely hot gas ejected by young, massive stars
ESO is a member of EIROforumw5, the publisher of Science in School.
Probing the cosmos across the EM spectrum is one of the scientific objectives of the European Space Agency (ESA; see box)w2, which currently has five missions in operation that are dedicated to astronomy (see Figure 5). In order of increasing energies, they are Planck (sub-millimetre and microwaves), Herschel (IR), Hubble Space Telescope (visible, as well as some IR and UV wavelengths), XMM-Newton (X-rays), and INTEGRAL (gamma and X-rays)w3.
In future Science in School articles, we will explore the EM spectrum in greater detail with help from ESA’s fleet of past and present space telescopes, which have contributed to reshaping our understanding of the Universe.
Mignone C, Pierce-Price D (2010) The ALMA Observatory: the sky is only one step away. Science in School 15: 44-49. www.scienceinschool.org/2010/issue15/alma
w2 – For more information about ESA, see: www.esa.int
w3 – For a spectacular view of the many different ‘colours’ of the Andromeda galaxy, as probed across the EM spectrum by various ESA missions, see: www.esa.int/export/esaSC/SEM5IUYGRMG_index_0.html
w4 – ESO is the world’s most productive astronomical observatory, with its headquarters in Garching near Munich, Germany, and its telescopes in Chile. To learn more about ESO, the VLT, ALMA and other ESO facilities, see: www.eso.org
w5 – To find out more about EIROforum, see: www.eiroforum.org
The Science@ESA vodcasts explore our Universe through the eyes of ESA's fleet of science spacecraft. Episode 1 (‘The full spectrum’) examines why we need to send telescopes into space and what they can tell us about the Universe. See: http://sci.esa.int/vodcast
To learn more about Earth’s atmosphere and the role – and loss – of ozone, see:
To see how physics teacher Alessio Bernadelli inspired his students about the EM spectrum by getting them to produce their own TV show on the subject, see Alessio’s blog (http://alessiobernardelli.wordpress.com) or use the direct link: http://tinyurl.com/42ow4a9
To find out how the wavelength at which a celestial object emits most of its light is related to the object’s temperature, see: http://sci.esa.int/jump.cfm?oid=48986
ESA has produced a wide range of freely available educational materials to support teachers in the classroom, which include printed materials, DVDs and online videos, teaching kits and websites. To see the full list, visit: www.esa.int/educationmaterials
To find out about all ESA education activities, see: www.esa.int/education
Claudia Mignone, Vitrociset Belgium for ESA – European Space Agency, is a science writer for ESA. She has a degree in astronomy from the University of Bologna, Italy, and a PhD in cosmology from the University of Heidelberg, Germany. Before joining ESA, she worked in the public outreach office of the European Southern Observatory (ESO).
Rebecca Barnes, HE Space Operations for ESA – European Space Agency, is the Education Officer for the ESA Science and Robotic Exploration Directorate. She has a degree in physics with astrophysics from the University of Leicester, UK, and previously worked in the education and space communications departments of the UK’s National Space Centre. To find out more about the education activities of the ESA Science and Robotic Exploration Directorate, contact Rebecca at SciEdu@esa.int
This article presents the reader with applications of the electromagnetic spectrum that are not usually considered when tackling this topic. Furthermore, it provides opportunities for teachers to engage their students and motivate further research into this fascinating topic.
The ESA vodcasts mentioned in the resource section are excellent material to engage learners in the topic of EM radiation. Teachers can also subscribe to receive the latest vodcasts.
Possible comprehension and extension questions include:
We normally associate the launch of astronomical telescopes with NASA. This article, however, makes it clear that Europe is also actively studying the skies – which should bring the topic closer to home for European students, and makes the science more relevant to them.
Angela Charles, Malta