(Electromagnetic spectrum with light highlighted. )
In its most common sense, light is a physical phenomenon identified as electromagnetic radiation that can produce a visual sensation. It consists of all the electromagnetic waves perceived by the human visual system, that is to say whose wavelengths, in the vacuum, are between 380 nm (violet) and 780 nm (red). Through an adaptation of living species to their environment, this region of the electromagnetic spectrum crosses the region where solar irradiance is greatest at the surface of the Earth.
The discipline that studies light is optics. Since the laws of light propagation are largely similar to those of other electromagnetic radiations, especially since their wavelengths are close to the visible spectrum, the optics often extend to other electromagnetic waves located in infrared and ultraviolet domains; that’s how we talk about black light, ultraviolet light or infrared light, which sometimes pushes to use the term visible light to avoid ambiguity. Physiological optics studies more particularly the perception of light by human beings. Photometry links physical measurements of electromagnetic radiation with human vision; colorimetry connects them to the perception of colors.
For the human being, light is indispensable to vision, and is an important part of well-being and social life. Lighting is an artistic and industrial specialty that is subject to legal standards. Light transports much of the solar energy to the earth’s surface and maintains the balance of the natural environment, with the regeneration of oxygen by the chlorophyll of plants.
Light has a strong symbolic value; to perceive objects before touching them, it is associated in all human cultures with knowledge.
Propagation and perception
The light travels in a straight line in any homogeneous transparent medium, especially vacuum or very dry air. It can, however, change course when moving from one medium to another. In the vacuum, the light moves at a strictly fixed speed and less quickly in the other media. The statement “the speed of light is constant” only makes sense “in a vacuum”, which is often implied. The light is a little slower in the air, and noticeably slower in the water. Fermat’s principle or Descartes’ laws make it possible to deduce the changes of trajectory of light as it passes from one medium to another according to its speed in each medium.
The light can also be decomposed (the beams take different directions according to their wavelength, and therefore according to their color for visible light) through different transparent media, because the speed can depend on the frequency. The light is only perceived by a receiver if it goes directly in its direction.
Photometry
From the physical point of view, it is quite irrelevant whether radiation is visible or not. The evaluation of the effect of electromagnetic radiation on the illumination is the object of photometry. These studies, undertaken since the seventeenth century, have resulted in the establishment of curves or spectral luminous efficiency tables. One can thus, knowing the power of a radiation for each wavelength, calculate its luminous effect. More practically, with a sensor provided with a suitable (optical) filter, one can measure a luminous flux or illuminance.
Color
When the light level is sufficient (photopic vision), the human being distinguishes colors, corresponding to the spectral distribution of the lights that reach him. The vision is a complex perception, a cognitive activity in which several brain areas collaborate, comparing the sensations to those recorded in the memory, with several effects in return. In particular, the color vision adapts to the ambient lighting, so as to attribute to the objects a color, even if, due to a variation of the light, the retina receives different radiations.
The human being is trichromate, his eye has three types of receptors, whose spectral sensitivity is different; the differences between their answers is at the base of the perception of colors. Therefore, two lights of very different spectral composition can be perceived to be of the same color, if their influence on the three types of receivers is equal. It is said that the lights are metameric. It is this particularity that is exploited in photography and color printing, as well as in television and computer screens. With three well-chosen colors, called primary colors, one can create, either by additive synthesis or by subtractive synthesis, the perception of many colors. The study of color perception, according to the physical characteristics of the light radiation, is the object of colorimetry.
Physical description of the light
Waves and corpuscles
In 1678, Christian Huygens proposes a wave theory of light, published in 1690 in his Treatise on Light.
In 1801, Thomas Young experimented with diffraction and interference of light.
In 1821, Augustin Fresnel states that the wave concept of light alone is capable of convincingly explaining all polarization phenomena by establishing the transversal nature of light waves.
In 1850, Léon Foucault makes the wave theory on the Newtonian corpuscular theory prevail with his experiment on the speed of propagation of light.
It was not until the work of James Clerk Maxwell to explain the wave phenomenon: he published in 1873 a treatise on electromagnetic waves, defining light as a wave that propagates in the form of a radiation which is the small part of the set of electromagnetic radiation that coincides with the region of maximum energy of solar radiation. In this radiation, the limits of the visible spectrum are imprecise. The spectral luminous efficiency varies a little from one species to another. Some birds and insects distinguish ultraviolet, invisible to humans. The infrared sufficiently intense give a feeling of warmth on his skin. From the point of view of physics, whether the radiation is perceived or not does not matter; the electromagnetic spectrum extends, beyond the infrared, towards the radio waves, and beyond the ultraviolet, towards the X-rays and gamma.
Maxwell’s equations allow us to develop a general theory of electromagnetism. They therefore make it possible to explain both the propagation of light and the operation of an electromagnet. For simple cases, the laws of geometrical optics describe well the behavior of waves (it is shown that these laws are a special case of Maxwell’s equations). This classic description is most used to explain the propagation of light, including complicated phenomena such as the formation of a rainbow or Young’s slits.
Newton had developed a purely corpuscular theory of light. It was rejected with the demonstration of interference phenomena (in some cases, adding two sources of light gives darkness, which can not be explained by a corpuscular theory).
Twentieth-century physics has shown that the energy transported by light is quantified. The photon is the quantum of energy (the smallest amount of energy, indivisible), which is also a particle. Quantum mechanics studies the wave-particle duality. The relevant model depends on the conditions of the study. If we consider the travel of a single photon, we can only know a probability of arrival at a point. On a very large number of photons, each place of arrival is illuminated with an intensity proportional to the probability … which corresponds to the result of the wave theory.
Speed
In 1670, Ole Christensen Rømer indirectly measures the speed of light by observing Io’s orbit offsets. Later in 1849, Hippolyte Fizeau directly measures the speed of light with a beam reflected by a distant mirror and passing through a gear wheel. The speed of light in the vacuum, noted c, is a constant of physics. This property was induced from the Michelson and Morley interferometry experiment and was clearly stated by Albert Einstein in 1905.
This is the maximum speed allowed for any movement of anything that carries information or energy, according to the theory of relativity. Other units are defined from the speed of light. In particular, the meter is defined so that the speed of light in the vacuum is 299,792,458 m/s. As a result, the speed of light is accurate because it no longer depends on a measurement (imprecise and subject to change with measurement progress).
Addition of velocities and speed
The law of addition of velocities v’ = V + v is nearly true for low speeds with respect to the speed of light. From the point of view of classical physics, a traveler walking in a train has, relative to the ground, a speed equal to that of the train plus (vectorially) its own speed of walking in the train. And one writes d = (V + v)t = Vt + vt = the distance traveled by the train + the distance traveled in the train = the distance traveled by the traveler with respect to the ground in the time t which is classically the same in the train and on the ground, which implies the classical law of adding speeds. This is only an approximation, which becomes less and less accurate as the considered speed v increases.
A photon goes at the same speed c either in relation to the ground or in relation to the train! The law of addition of the velocities is only an approximation of the so-called law of transformation on Lorentz velocities (sometimes called velocity addition, or more correctly velocity composition law). This result is one of the characteristics of the special relativity; the law of composition of the velocities resulting from the Lorentz mathematical transformations gives to the limit of the low velocities (with respect to the velocity c) the same results as the transformations of Galileo.
In materials
The speed of light is not always the same in all environments and in all conditions. The differences in speed observed between two media are related to the refractive index, which characterizes the responses of media to the crossing of an electromagnetic wave.
The difference between the speed of light in the vacuum and the speed of light in the air is very small (less than 1%), which allowed us to speak in general about speed of light instead of speed of light in vacuum. However, in the condensed matter, a light wave can be considerably slowed down (for example by 25% in water). Physicists have even succeeded in slowing the light propagation by electromagnetically induced transparency up to a speed of a few meters per second in extreme cases.
In the International System (SI)
Currently, most units of the International System are defined from the speed of light. A speed being the quotient of a length by a duration, we can thus define a distance as being the product of a duration by a speed (in this case c), or a duration like the division of a distance by c.
The second is defined in the International System by a luminous phenomenon: it is the duration of 9.192.631.770 periods of the radiation corresponding to the transition between the two hyper-fine levels of the ground state of the atom cesium 133.
The meter is the unit of the length in the International System. Nowadays, it is defined as the distance traveled by the light, in a vacuum, in 1/299.792.458 of a second. This is a conventional definition, as any change in the definition of the second would have a direct impact on the length of the meter. With the current definition of the second, the meter is therefore equal to 9.192.631.770.299.792.458 times the wavelength of the chosen radiation.
We can also say that the speed of light in a vacuum is precisely 299.792.458 m·s-1: there is not the least uncertainty about this value, except for the uncertainty in the definition of second.
The meter, with its submultiples or multiples (millimeter, kilometer), is very convenient for measuring distances on Earth; on the other hand, for astronomers, it is too short and unsuitable (since astronomers observe practically only light). Indeed, the Moon, the star closest to us, is about 380.000.000 meters from us and the Sun, the nearest star, is about 150.000.000.000 meters. With the principle described previously (distance = c x duration), the light year is defined as the distance that light travels in one year. Thus the Sun is only 8,32 light-minutes from us; and the Moon is only a little more than a second-light. The light-year is exactly 9.460.730.472.580.800 meters (or about ten million of billion meters, or 1016 m).
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