![]() It is in this visible domain that solar radiation reaches its maximum intensity, in the vicinity of λ = 0.5 µm. The visible domain corresponds to the very narrow part of the spectrum perceptible by our eye, between λ = 0.4 µm for violet light and λ = 0.8 µm for red light (Figure 3). This spectrum has a considerable range and varies by 10 18 orders of magnitude between the shortest wavelengths, those of γ rays (10 -14 m<λ<10 -12 m) and the longest, those of radio waves (10-1 m<λ< 10 4 m), as shown in Figure 2. The electromagnetic spectrum represents the distribution of all electromagnetic waves according to their wavelength or frequency. A wave can also be characterized by its frequency ν which is none other than the inverse of the period ν = 1/T and is expressed in hertz (Hz) 1 Hz corresponds to a period T = 1s. It is expressed either in metres (m) or in one of its submultiples: micrometers (1 µm = 10 -6 m), or nanometers (1 nm =10 -9 m), depending on the characteristic scale of the phenomenon. The wavelength is the product of this period with the speed: : λ = cT. At a given point these fields cyclically oscillate with a period T, which is expressed in seconds. The wavelength λ is the distance between two successive maximums of the fields. In vacuum this velocity is c = 3×10 8 m/s (meter per second). The propagation of electromagnetic waves is carried out at a speed, often called celerity, which depends on the considered environment. These two fields are perpendicular to each other and propagate in the third z-direction, orthogonal to their directions, as shown in Figure 1. Such a wave includes both an electric field, noted E, and a magnetic field, noted B, oscillating at the same frequency. Light is a superposition of electromagnetic waves. ![]() Representation of an electromagnetic wave propagating in the Oz direction at a given time. To do this, let us recall the constitution of light and first of all the nature of an electromagnetic wave and its propagation mode. Why is the sky blue on clear days? Why are the sunsets red? Why is the night black? All these questions seem naive but their answers are not so simple and involve physical phenomena that we will try to clarify. Some phenomena are so familiar to us that we often take them for granted. The Earth’s atmosphere (read the article on The Earth’s Atmosphere and Gaseous Envelope), visible in the header photograph, imposes the colour of the sky and gives rise to surprising light phenomena such as mirages and rainbows. ![]() These mechanisms are at the origin of the different colours of the celestial vault and spectacular phenomena such as Brocken’s spectrum. Why is the sky blue, clouds white, grey, or black? Why are the sunsets red? Why is the night black? Where do rainbows come from? These phenomena are largely explained by the interaction, absorption and scattering of light from the Sun with the components of the Earth’s atmosphere: water droplets of various sizes, gas molecules, dust, etc.
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