Light is one of the basic phenomenawhich humankind has encountered from time immemorial. The main and the most important source of light on Earth is the Sun. Another common sources of light that have existed throughout the whole history of our planet can be various exothermic chemical reactions like burning, or physical processes connected to the release of energy leading to emitting light radiation (discharges, geothermal effects etc.).

Understanding of the “term” light

Although light has an immense and fundamental significance for the life on Earth, an acceptable explanation of its substance for humankind was an unsolvable problem for a long time. To this day, an accurate understanding of the term “light” from the physics point of view is still very difficult.Philosophers and scientists have been trying to describe the phenomenon of light and its characteristics since a long time ago. Hellenic philosopher Plato assumed that humans´ eyes were the active source of light. A mathematician and a geometer Euclid shared the same opinion with Plato. This relatively spread concept naturally collided with a quite evident dispute in the question “why in that case we cannot see in the dark”. An atomist Democritus, in compliance with his idea that all the objects in the world consist of very small particles, further impartible atoms, assumed that light as well was a stream of particles that was radiated by every visible object. On the contrary, the philosopher Aristotle, whose extensive encyclopaedic work laid the foundations of many sciences and who was a fundamental authority not only in the ancient history but also throughout the Middle Ages, did not agree with the atomistic point of view and assumed that light itself was not a body nor was emitted by a body but it spread through space in the same way as waves did on water surface. It is very interesting that both fundamental explanatory concepts of the term “light” already appeared in Ancient Greece, i.e. Democritus´s Corpuscular theory of light (particular) and Aristotle´s Wave theory of light. We will come back to both theories throughout the work.

Newton and light

Modern philosophy, emerging modern biology and physics came back to the term of “light”. Isaac Newton, an English philosopher, biologist, physicist, and mathematician, in particular, described properties of light in details in his work “Optics: or, A treatise of the reflexions, refractions, inflexions and colours of light” (shortened Optics), published in 1704. Newton in his work assumes that light is a current of very small particles spreading out from luminous bodies and on the basis of this assumption he explains phenomena of geometrical optics as reflection, refraction and dispersion of light. Newton´s corpuscular theory of light however did not manage to properly explain all the observed optical phenomena, for example interference (combining) of light, diffraction of light by an aperture or polarization. At the same time as Newton´s corpuscular theory of light, a wave theory of light appears. Christiaan Huygens, a Dutch physicist, mathematician and an astronomer is its author. In 1678, he described light as a wave motion and with the help of his wave theory he successfully explained most of light´s specific properties. He also reintroduced the ancient term “aether”. The term determined a hypothetical omnipresent substance within light spreads in the same way as an ordinary mechanical wave motion spreads through common mass. Later in the 19thcentury it was assumed that other types of newly discovered electromagnetic wave motion spread in aether.

Electromagnetic wave

Work of a Scottish physicist James Clerk Maxwell closely relates to the electromagnetic wave motion. He described the connection between electricity and magnetism in the 19th century. From the four Maxwell´s elemental equations it derives, that light is only one of the types of electromagnetic radiation. It is a very narrow band of wavelengths. Depending on the colour, visible light covers the range of wavelengths from 400 nm–760 nm; electromagnetic radiation in general can have wavelengths range from 10–12 m (gamma rays) to 103 m (long radio waves). Out of the four Maxwell´s elemental equations also derives that there is no need for aether for spreading out light, it can spread out in vacuum as well as in a medium such as air or water.

Photoelectric effect

At the beginning of the 20th century a photoelectric effect was discovered, whose experimental behaviour did not respond to that behaviour of light predicted by Maxwell´s theory of electromagnetic radiation. This effect was explained in 1905 by Albert Einstein, but surprisingly, based on corpuscular theory of light that was at that time already considered to be completely surpassed. In the new form of this theory light particles, photons, can only exist with clearly defined energy, as energy quantum. Photons are special particles that cannot exist in rest, they always move at the speed of light (within a vacuum this speed is c = 3 ∙ 108 m/s).

There is a clear connection between energy of photons and colour of light. Since the colour of light corresponds with the wavelength of given electromagnetic radiation it is possible to match a wavelength to the photon (particle) as well. This fact is absolutely in contrary to the experience of classical physics. However, in the course of the 20th century along with the advancement of quantum mechanics physicists come to a conclusion that every physical object has both particle and wave motion properties concurrently. For example, an electron can be in almost all cases considered a typical particle, but under some circumstances in the micro world, it has wave properties as well. Photon as an elementary quantum of light, or rather electromagnetic radiation in general, has also both properties of a particle and properties of wave. Some of the light effects can be better explained by the corpuscular theory when we see light as a stream of particles, other effects can be explained by the wave theory, when we consider light to be a wave.Luis de Broglie, a French physicist, generalized this thought for all mass in quantum mechanics when he postulated common principle of particle duality and wave motion

Light refraction

Among the properties of light, which can be well-explained by the wave theory of light, belongs the light refraction when passing between two mediums of optically different density. Light refraction is a special example of refraction of wave, which is its common property when it passes across the boundary between two media where the wave motion has different phase velocity. The refraction of wave derives from Huygens principle which describes wave propagation by wave fronts. The specific result of light refraction is dispersion of light. White light, which consists of lights of different wavelengths, therefore the colours as well, is separated into its constituent colours when refracted and a spectrum of colours is revealed. It is caused by a different wavelength of each colour that moves at a different speed through a medium, causing the light to refract differently. In the spectrum, it is possible to observe colours according to their gradually decreasing wavelengths of a given light from red colour having the longest wavelength, to orange, yellow, green, blue, indigo and violet which has the shortest wavelength. These seven colours formed by light dispersion, were named by Newton but it is important to realise that there are an infinite range of colour shades.

The dispersion of light is commonly demonstrated on light passing through a prism. Here a double refraction of light occurs – on the wall which light enters the prism and on the wall that it leaves the prism – that is why the dispersion is clearer. The rainbow is the typical phenomenon where the light dispersion happens in nature. The refraction and dispersion of the white sun light in the rainbow happen when light enters the raindropand when it leaves it. The rainbow occurs while it rains and the sun shines at the same time. However, it can also occur in water spray above a waterfall or when sprinkling with a garden hose.

The sun light, just as light that is emitted by other incandescent sources of light (halogen and ordinary light bulbs, candles) has a continuous spectrum and contains light of all wavelengths. According to the temperature of a luminous body, the wavelength, which the body emits the maximum of energy on, can be moved either to the red or violet end of the spectrum. That is why the sun light is white, the light of a bulb yellow and the candle light orange. However, all of these sources still have a continuous spectrum. It is interesting that other sources can have discontinuous spectrum (line or band) and despite that our eye still perceives their colour as white. (This is connected to the physiology of human colour vision.) Examples of the sources with discontinuous spectrum are energy saving bulbs or LCD displays and monitors.Other light properties, particularly its linear propagation and reflection, can be on the contrary, better explained by the corpuscular (particle) theory of light. Thinking of the image of a light particlecurrent makes the principle of rectilinear propagation of light completely obvious. Clearly the same is the reflexion of light from a boundary (typically from a mirror), when it is possible to demonstrate the reflection in a mechanical way as a perfectly elastic collision in which the total kinetic energy and particle movement remain the same. Light reflects from mirror surfaces according to the same laws as a billiard ball does from a billiard table cushions.

Properties of light

Light during its propagation transmits energy. This fact derives from the corpuscular (particle) theory of light, when in a mechanistic concept every light particle has its speed, weight and also its kinetic energy and momentum. Here attention must be drawn to the fact that photons cannot be at rest so there is no point in using rest mass when referring to them. They constantly move at the speed of light, they have accurately defined quantum energy (which is connected to their wavelength) therefore, they have weight as followed from the special theory of relativity. Transmission of energy by propagating light can also be explained by the wave theory of light. Light, according to Maxwell is only one of the types of electromagnetic wave motion and every electromagnetic wave motion when moving through space transmits energy. Transmission of energy by wave motion is again possible to simply view in a mechanistic way. Propagation of energy by wave motion can be simply viewed in a mechanistic way. Likewise, mechanical waves transfer energy through space in which it travels, as the disturbance passes from one oscillating point to another

If we observe energy emitted by an incandescent source of light (the Sun, light bulbs), it is necessary to realise that such sources do not only emit light in the stated narrow range of wavelengths of 400 nm – 760 nm but they also emit infrared thermal radiation and to a certain extent, they also emit ultraviolet radiation. For example, an ordinary light bulb emits 90–95 % of energy in the form of thermal radiation and the rest as light radiation. On the contrary, cold sources of light (i.e. LED bulb) emit almost all of their energy in the form of visible light

Black body

For the need of accurate calculation of energy that is emitted or absorbed by incandescent source of electromagnetic radiation a term blackbody is being introduced. It is an idealized physical body that absorbs all incident electromagnetic radiation that falls on its surface. All bodies on the contrary reflect off a part of luminous energy falling on their surfaces, therefore they always absorb less energy than a blackbody. Bodies with either white or mirror-like surfaces absorb the least energy. The black body is also the ideal radiator as it emits the most energy at given temperature compared to all other possible bodies. The same goes for white bodies or bodies with mirror-like surfaces, which emit little energy (metal, lustrous thermal blanket foils of rescuers).