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.
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.
Probing the cosmos across the EM spectrum is one of the scientific objectives of the European Space Agency, 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).
The Electromagnetic Spectrum
~Abhimit Suman
(Batch of 2020-21)
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