Modern physical optics considers light as a type electromagnetic waves, perceived by the human eye. In other words, we can say that light is visible electromagnetic radiation.
Visible light
As is known, electromagnetic waves differ in frequency and wavelength. And depending on these values, electromagnetic radiation is divided into frequency ranges.
Outside physical optics The concept of “light” also includes electromagnetic waves that are not visible to the human eye, in infrared range with a wavelength of 1 mm - 780 nm and a frequency of 300 GHz - 429 THz and in the ultraviolet range with a wavelength of 380 - 10 nm and a frequency of 7.5 10 14 Hz - 3 10 16 Hz.
Infrared, visible and ultraviolet radiation are called optical spectrum region. Upper limit The optical range is the long-wave limit of infrared radiation, and the lower one is the short-wave limit of ultraviolet radiation. So the range optical radiation- from 1 mm to 10 nm.
How does light come about? It turns out that it is formed as a result of processes occurring inside atoms when their state changes. This creates a stream of particles called photons. They have no mass, but they have energy.
It turns out that light simultaneously has the properties of an electromagnetic wave and the properties of discrete particles - photons.
Sources of light
Any body that emits electromagnetic waves with a frequency in the range visible light, can be called a light source. All light sources are divided into natural, created by nature itself, and artificial, created by people.
The most important natural source of light on Earth is, of course, the Sun. It gives us not only light, but also warmth. Thanks to energy sunlight there is life on our planet. Light is emitted by the Moon, stars, comets and others cosmic bodies. Sources natural light there can be not only bodies, but also natural phenomena. During a thunderstorm, we see with what powerful light a flash of lightning illuminates everything around. Auroras, luminous living organisms, minerals, etc. - this is also natural springs Sveta.
The very first and oldest artificial light source can be called fire fire. Later, people learned to use other types of fuel and create portable light sources: candles, torches, oil lamps, gas lanterns, etc. All these sources were based on combustion and, together with light, emitted a large number of heat.
With the invention of electricity, light bulbs appeared, which are still used by people today as light sources.
Geometric optics
The propagation of light in a transparent medium, its reflection from specularly reflected surfaces, refraction at the boundary of two transparent media occurs according to certain laws, which are studied by geometric optics.
To study various light phenomena in geometric optics, concepts such as a point light source and a light beam are used.
The basic concept of geometric optics is light beam .
An ordinary lamp distributes light evenly in all directions. Let us cover this lamp with an opaque material so that the light emitted by it can pass only through a small narrow hole. A narrow light flux will go through it, directed along a straight line. This line along which the light beam propagates is called a light ray. The direction of this beam does not depend on its transverse dimensions.
Candles, lanterns, lamps and other light sources have quite big sizes compared to the distance over which their light travels. They are called extended light sources . Point light source a source is considered to be the size of which can be neglected compared to the distance to which this light reaches. For example, space star, which is actually enormous in size, can be considered a point source of light, since the distance over which this light spreads is enormous compared to the size of the star itself.
Let's consider the basic laws of geometric optics.
Law of rectilinear propagation of light
In a transparent homogeneous medium, light travels in a straight line. The proof of this law is the experience in which light from point source passes through a small hole in the screen. As a result, a narrow light beam is formed, and in a plane located behind the screen parallel to it, a regular circle of light appears with its center on a straight line along which the light propagates.
Let's place a small object between the light source and the screen. On the screen we will see the shadow of this object. Shadow- this is the area where the light beam does not reach. Its appearance is explained by the rectilinear propagation of light. If the light source is point-like, then only a shadow is formed. If its dimensions are quite large compared to the distance to the object, then shadow and penumbra are created. Indeed, in this case, light rays emanate from each point of the source. Some of them, falling into the shadow area, highlight its edges, and thereby create penumbra - the area into which light rays partially fall.
The law of rectilinear propagation explains the nature of solar and lunar eclipse. Solar eclipse occurs when the Moon is located between the Sun and the Earth, and the shadow of the Moon falls on the Earth.
The law of rectilinear propagation of light was used by the ancient Greeks when installing columns. If the columns are placed strictly in a straight line, then the closest one will visually cover all the others.
Law of Light Reflection
If on the way light beam When a reflective surface is encountered, the light beam changes its direction. The incident and reflected rays and the normal (perpendicular) to the reflecting surface, restored at the point of incidence, lie in the same plane. The angle between the rays is divided by this normal into two equal parts. The most common formulation of the law of reflection is: “ Angle of incidence equal to angle reflections" But this definition does not indicate the direction of the reflected ray. Meanwhile, the reflected beam will go in the opposite direction to the incident beam.
If the size of the surface irregularities is smaller than the length of the light wave, then the rays incident in a parallel stream will be reflected specularly and will also travel in parallel streams.
If the dimensions of the irregularities exceed the wavelength, then narrow bun will be scattered, and the reflected rays will go in different directions. This reflection is called diffuse, or absent-minded. But, despite the random scattering, the law of reflection is satisfied in this case as well. For any ray, the angle of incidence and the angle of reflection will be equal.
Law of light refraction
Dip the pencil into a cup of water. Visually, it seems to us that it seems to have broken in two on the surface of the water. In fact, nothing happened to the pencil. The reason is that a ray of light hits the surface of the water at one angle, and goes deeper at another. Because of this, the size and location of physical bodies are distorted.
Changing the direction of a light beam at the interface between two media transparent to light waves called refraction Sveta.
The law that describes the refraction of light waves is called Snell's law(Snell or Snell) named after its author - the Dutch mathematician Willebrord Snellius, who discovered it in 1621.
According to this law, the angle of incidence of light on the interface and the angle of refraction are related by the relation:
n 1 sinƟ 1 = n 2 sinƟ 2 ,
or sin Ɵ 1 / sin Ɵ 2 = n 2 / n 1 ,
Where n 1 - refractive index of the medium from which light falls on the interface;
Ɵ 1 - the angle between the light beam incident on the interface and the normal to this surface;
n 2 - the refractive index of the medium into which light enters after the interface;
Ɵ 2 - the angle between the ray passing the interface and the normal to this surface.
Refractive index of the medium is the ratio of the speed of light in a vacuum to its speed in a given medium:
n = c/v
The more it differs from unity, the greater will be the angle of deflection of the light beam when passing from vacuum to medium.
Attitude n 2 / n 1 called relative indicator refraction .
A ray of light entering a denser medium forms a smaller angle with the normal to this surface, that is, it is refracted downward. But in reality, it seems that this angle, on the contrary, is larger than the angle of incidence. As a result of this, we observe a distortion in the size, shape and location of objects. Objects in water seem larger to us than they actually are and located higher. Thus, swimmers often make mistakes when assessing the depth of a reservoir. They see the bottom raised, and the depth seems shallower to them.
Due to the refraction of sunlight in the atmosphere, we observe the sunrise a little earlier and the sunset a little later than these phenomena would occur if there were no atmosphere.
Lenses for photo and movie cameras, microscopes, telescopes, binoculars and others are built on the basis of the phenomenon of refraction. optical instruments, which contain optical lenses or prisms.
When light passes from a denser medium to a less dense one (for example, from water to air), one can observe total internal reflection of a light beam . It occurs when the angle of incidence is equal to a certain value called limit angle full internal reflection . In this case, the incident rays are completely reflected from the interface. The refracted rays disappear completely.
This phenomenon is used in fiber LEDs, which are made of optically transparent material. They are very thin threads. The light entering them is completely reflected from the inner side surfaces and spreads over long distances.
Geometric optics considers the properties of light without taking it into account wave theory And quantum phenomena. Of course, it cannot accurately describe optical phenomena. But since its laws are much simpler compared to the generalizing wave laws, it is widely used in the calculation of optical systems.
One of the characteristics of light is its color, which for monochromatic radiation is determined by the wavelength, and for complex radiation - by its spectral composition.
Light can propagate even in the absence of matter, that is, in a vacuum. In this case, the presence of matter affects the speed of light propagation.
Each energy quantity has a corresponding analog – a light photometric quantity. Light quantities differ from energy quantities in that they evaluate light by its ability to evoke visual sensations in a person. Light analogues of the energy quantities listed above are luminous energy, luminous flux, luminous intensity, brightness, luminosity and illumination.
Taking into account the dependence of visual sensations on the wavelength of light by light quantities leads to the fact that for the same values, for example, the energy transferred by green and violet light, the light energy transferred in the first case will be significantly higher than in the second. This result is in full agreement with the fact that the sensitivity of the human eye to green light is higher than to violet light.
Speed of light
The speed of light in a vacuum is determined to be exactly 299,792,458 m/s (about 300,000 km per second). The fixed value of the speed of light in SI is due to the fact that the meter is currently defined in terms of the speed of light. All types of electromagnetic radiation are believed to travel at exactly the same speed in a vacuum.
Various physicists have attempted to measure the speed of light throughout history. Galileo tried to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was carried out by Ole Römer, a Danish physicist, in 1676. Using a telescope, Roemer observed the movements of Jupiter and one of its moons, Io. Noting differences in the apparent period of Io's orbit, he calculated that the light took about 22 minutes to cross the diameter of Earth's orbit. However, its size was not known at the time. If Roemer had known the diameter of the Earth's orbit, he would have obtained a speed value of 227,000,000 m/s.
Another, more exact way, measurements of the speed of light were carried out in Europe by Hippolyte Fizeau in 1849. Fizeau directs a beam of light into a mirror several kilometers away. A rotating gear wheel was placed in the path of a light beam that traveled from a source to a mirror and then returned to its source. Fizeau discovered that at a certain speed of rotation, the beam would pass through one gap in the wheel on the way and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the speed of rotation, Fizeau was able to calculate the speed of light 313,000,000 m/s.
Leon Foucault used an experiment that used a rotating mirror to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments to determine the speed of light from 1877 until his death in 1931. He improved on Foucault's method in 1926 by using improved rotating mirrors to measure the time it took light to travel from Mount Wilson to Mount San Antonio in California. Accurate measurements given a speed of 299,796,000 m/s.
The effective speed of light in various transparent substances containing ordinary matter is less than in a vacuum. For example, the speed of light in water is about 3/4 of that in a vacuum. However, the slowing down of processes in matter is believed to occur not from the actual slowing down of light particles, but from their absorption and re-emission by charged particles in matter.
As an extreme example of light slowing down, two independent groups of physicists were able to "completely stop" light by passing it through a rubidium-based Bose-Einstein condensate. However, the word "stop" in these experiments only refers to light stored in excited states of atoms, and then re-emitted into an arbitrary more late time, as radiation stimulated by the second laser pulse. At the time when the light "stopped", it ceased to be light.
Optical properties of light
The study of light and the interaction of light and matter is called optics. Observation and study optical phenomena such as rainbow and northern lights allow us to shed light on the nature of light.
Refraction
An example of light refraction. The straw appears curved due to the refraction of light as it enters the liquid from the air.
Refraction of light is the change in the direction of propagation of light (light rays) when passing through the interface between two different transparent media. It is described by Snell's law:
where is the angle between the ray and the normal to the surface in the first medium, is the angle between the ray and the normal to the surface in second environment, and and are the refractive indices of the first and second medium, respectively. Moreover, for vacuum and in the case of transparent media.
When a beam of light crosses the boundary between a vacuum and another medium, or between two different environments, the wavelength of light changes, but the frequency remains the same. If the light beam is not orthogonal (or rather normal) to the boundary, changing the wavelength causes the beam to change direction. This change in direction is the refraction of light.
The refraction of light by lenses is often used to control light in a way that changes visible size images, such as in magnifying glasses, glasses, contact lenses, microscopes and telescopes.
Sources of light
Light is created in many physical processes, in which charged particles participate. The most important is thermal radiation, which has a continuous spectrum with a maximum depending on the temperature of the source. In particular, the solar radiation is close to thermal radiation an absolutely black body heated to about 6000 K, with about 40% solar radiation lies in the visible range, and the maximum power distribution across the spectrum is located near 550 nm ( green color). Other processes that are sources of light:
- transitions to electronic shells atoms and molecules from one level to another (these processes give line spectrum and include both spontaneous emission- in gas-discharge lamps, LEDs, etc. - and stimulated emission in lasers);
- processes associated with the acceleration and deceleration of charged particles (synchrotron radiation, cyclotron radiation, bremsstrahlung);
- Cherenkov radiation when a charged particle moves at a speed exceeding the phase speed of light in a given medium;
- different types of luminescence:
- chemiluminescence (in living organisms it is called bioluminescence)
- scintillation
IN applied sciences Accurate characterization of the spectrum is important. Particularly important following types sources:
- Source A
- Source B
- Source C
- Source D 65
Fluorescent lamps Available in different light ranges, including:
- White light lamps (color temperature 3500),
- Cool white light lamps (color temperature 4300 K)
Radiometry and light measurements
To one of the most important and demanded by science and practice characteristics of light, like any other physical object, relate energy characteristics. The measurement and study of such characteristics, expressed in energy photometric quantities, is the subject of a branch of photometry called “optical radiation radiometry”. Thus, radiometry studies light without regard to the properties of human vision.
On the other hand, light plays a special role in human life, providing him with most of the information about the world around him that is necessary for life. This happens due to the presence of human organs of vision - eyes. This implies the need to measure such characteristics of light by which one could judge its ability to excite visual sensations. The mentioned characteristics are expressed in light photometric quantities, and their measurements and studies are the subject of studies in another section of photometry - “ light measurements» .
Light and energy quantities are related to each other using the relative spectral luminous efficiency of monochromatic radiation for daylight vision, having the meaning of the relative spectral sensitivity of the average human eye adapted to daylight vision. For monochromatic radiation with wavelength , the relation connecting an arbitrary light quantity with its corresponding energy quantity is written in SI as:
IN general case, when no restrictions are imposed on the distribution of radiation energy across the spectrum, this relationship takes the form:
Light quantities belong to the class of reduced photometric quantities, to which other systems also belong photometric quantities. However, only light quantities are legalized within the SI framework and only for them are special units of measurement defined in the SI.
Light pressure
Light exerts physical pressure on objects in its path - a phenomenon that cannot be derived from Maxwell's equations, but can be easily explained in corpuscular theory, when photons collide with an obstacle and transfer their momentum. The pressure of light is equal to the power of the light beam divided by c, the speed of light. Due to the magnitude of c, the effect of light pressure is negligible for everyday objects. For example, a one milliwatt laser pointer produces a pressure of about 3.3 pN. An object illuminated in this way could be lifted, although for a 1 penny coin this would require about 30 billion 1-mW laser pointers. However, at the nanometer scale, the effect of light pressure is more significant, and the use of light pressure to control mechanisms and switch nanometer switches in integrated circuits is an active area of research.
History of theories of light in chronological order
Ancient Greece and Rome
In the early 19th century, Thomas Young's experiments with diffraction provided compelling evidence in favor of the wave theory. It was discovered that light is transverse waves and is characterized by polarization. Jung suggested that different colors correspond to different wavelengths. In 1817, Augustin Fresnel outlined his wave theory of light in a memoir for the Academy of Sciences. After the theory of electromagnetism was created, light was identified as electromagnetic waves. The victory of the wave theory was shaken at the end of the 19th century, when the Michelson-Morley experiment did not discover the ether. Waves require a medium in which they can propagate, but carefully designed experiments have not confirmed the existence of this medium. This led to Albert Einstein's creation special theory relativity. The nature of electromagnetic waves turned out to be more complex than simply the propagation of disturbances in matter. Max Planck's consideration of the problem of thermal equilibrium of an absolutely black body with its radiation led to the emergence of the idea of emitting light in portions - light quanta, which were called photons. Einstein's analysis of the photoelectric effect showed that the absorption of light energy also occurs by quanta.
With development quantum mechanics Louis de Broglie's idea of wave-particle duality was established, according to which light should have both wave properties, which explains its ability to diffraction and interference, and corpuscular properties, which explains its absorption and emission.
Wave and electromagnetic theories
Light in special relativity
Quantum theory
Wave-particle duality
Quantum electrodynamics
Perception of light by the eye
See the world we can only because light exists and man is able to perceive it. In turn, a person’s perception of electromagnetic radiation in the visible range of the spectrum occurs due to the fact that the retina of a person’s eye contains receptors that can respond to this radiation.
The retina of the human eye has two types of light-sensitive cells: rods and cones. Sticks have high sensitivity to light and function in low light conditions, thereby being responsible for night vision. However, the spectral dependence of sensitivity is the same for all rods, so rods cannot provide the ability to distinguish colors. Accordingly, the image obtained with their help is only black and white.
Cones have a relatively low sensitivity to light and provide a mechanism for daytime vision that operates only when high levels illumination At the same time, unlike rods, in the human retina there is not one, but three type of cones, differing from each other in the location of the maxima of their spectral sensitivity distributions. As a result, cones provide information not only about the intensity of light, but also about its spectral composition. Thanks to such information, a person develops color sensations.
The spectral composition of light uniquely determines its color perceived by humans. The converse statement, however, is not true: the same color can be obtained different ways. In the case of monochromatic light, the situation is simplified: the correspondence between the wavelength of light and its color becomes one-to-one. Data on such compliance are presented in the table.
Table of correspondence between frequencies of electromagnetic radiation and colors
| Color | Wavelength range, nm | Frequency range, THz | Photon energy range, eV |
|---|---|---|---|
| Violet | 380-440 | 790-680 | 2,82-3,26 |
| Blue | 440-485 | 680-620 | 2,56-2,82 |
| Blue | 485-500 | 620-600 | 2,48-2,56 |
| Green | 500-565 | 600-530 | 2,19-2,48 |
| Yellow | 565-590 | 530-510 | 2,10-2,19 |
“And God said: “Let there be light!” and there was light.” Everyone knows these words from the Bible and everyone understands: life without him is impossible. But what is light by its nature? What does it consist of and what properties does it have? What is visible and invisible light? We will talk about these and some other questions in the article.
About the role of light
Most information is usually perceived by a person through the eyes. All the variety of colors and shapes that are characteristic material world, opens up to him. And he can perceive through vision only what reflects a certain, so-called visible light. Light sources can be natural, such as the sun, or artificial, created by electricity. Thanks to such lighting, it became possible to work, relax - in a word, lead a full lifestyle at any time of the day.
Naturally, such an important aspect of life occupied the minds of many people who lived in different eras. Let's consider what light is, under different angles point of view, that is, from the standpoint various theories, which are adhered to by pundits today.
Light: definition (physics)
Aristotle, who asked this question, considered light certain action, which spread in the environment. A philosopher from Ancient Rome, Lucretius Car. He was sure that everything that exists in the world consists of the most fine particles- atoms. And light also has this structure.
In the seventeenth century, these views formed the basis of two theories:
- corpuscular;
- wave.
Today it is known that all bodies emit infrared light. Light sources emitting infrared rays, have a longer wavelength, but are weaker than red ones.
Heat is radiation in the infrared spectrum emanating from moving molecules. The higher their speed, the greater the radiation, and such an object becomes warmer.
Ultraviolet
As soon as they opened infrared radiation, Wilhelm Ritter, German physicist, began to study the opposite side spectrum The wavelength here turned out to be shorter than that purple. He noticed how the silver chloride turned black behind the violet. And this happened faster than the wavelength of visible light. It turned out that such radiation occurs when the electrons on the external atomic shells. Glass is capable of absorbing ultraviolet radiation, so quartz lenses were used in the studies.
Radiation is absorbed by human and animal skin, as well as by the upper plant tissues. Small doses of ultraviolet radiation can have a beneficial effect on well-being, strengthening the immune system and creating vitamin D. But large doses can cause skin burns and damage the eyes, and too large doses can even have a carcinogenic effect.
Application of ultraviolet
Conclusion
If we take into account the negligible spectrum of visible light, it becomes clear that the optical range has been studied very poorly by humans. One of the reasons for this approach is increased interest people to what is visible to the eye.
But because of this, understanding remains low. The whole cosmos is permeated electromagnetic radiation. More often than not, people not only don’t see them, but also don’t feel them. But if the energy of these spectra increases, they can cause illness and even become deadly.
When studying the invisible spectrum, some, as they are called, become clear mystical phenomena. For example, ball lightning. It happens that they appear as if out of nowhere and suddenly disappear. In fact, the transition from the invisible range to the visible and back is simply carried out.
If you use different cameras when photographing the sky during a thunderstorm, you can sometimes capture the transition of plasmoids, their appearance in lightning, and changes occurring in the lightning themselves.
Around us is a completely unknown world, which looks different from what we are used to seeing. The well-known statement “Until I see it with my own eyes, I won’t believe it” has long lost its relevance. Radio, television, cellular communications and the like have long proven that if we don’t see something, this does not mean at all that it does not exist.
Once upon a time, in ancient times, people believed that our ability to see was due to certain rays emanating from the eyes and, as it were, “feeling” the surface of objects. No matter how funny such a concept may seem today, think about it - do you know what light is? Where does it come from? How we perceive it and why various items do they have different colors?
Turn on the light bulb and place your hand near it. You will feel the heat emanating from the light bulb. Accordingly, light is radiation. All radiation carries energy, but not all radiation can be perceived visually. Let us conclude that light is visible radiation.
Properties of light
It has been experimentally established that light has electromagnetic nature, so we can supplement our definition in the following way: Light is visible electromagnetic radiation.
Light can pass through transparent bodies and substances. Therefore, the light of the sun penetrates to us through the atmosphere, although the light is refracted. And when encountering opaque objects, light is reflected from them, and we can perceive this reflected light with the eye, and thus we see.
Some of the light is absorbed by objects, and they heat up. Dark objects heat up more than light ones, respectively most of of light is absorbed by them, and less is reflected. That's why these objects look dark to us.
Black objects absorb the most light. That is why in the summer heat you should not wear black clothes, because you can get heatstroke. For the same reason, in the summer, mothers always wear light-colored hats for their children, which heat up much less than hair that has a darker color.
Sources of light
The bodies from which light comes are called light sources. There are natural and artificial sources Sveta. The most famous natural source of light to absolutely all inhabitants of our planet is the Sun.
The sun is not only a source of visible light, but also heat, due to which life on Earth is possible. Other natural springs the lights are the stars, atmospheric phenomena such as lightning, living things such as fireflies, and so on.
Thanks to man, there are also artificial sources. Previously, for people, the main source of light in the dark was fire: candles, torches, gas burners, and so on. Nowadays the most common are electrical sources Sveta. Moreover, they are in turn divided into thermal (incandescent lamps) and fluorescent (fluorescent lamps, gas light lamps).
Spread of Light
Another property of light is its linear propagation. Light cannot bend around obstacles, so a shadow forms behind an opaque object. The shadow is often not completely black because various reflected and scattered rays of light from other objects fall there.
« Light” refers to those categories that seem most familiar, understandable and simple, but in fact turn out to be the most complex. Generally speaking, throughout the development of physics, ideas about what light is have repeatedly changed dramatically.
IN ancient world opinions about light were very different. In the Newtonian era to a greater extent Geometric optics and a corpuscular view of light were developed, although wave concepts of light also arose (Huygens' principle). With the discovery of the phenomena of interference and diffraction, priority shifted to the wave theory of light, and within Maxwell’s framework it turned out that light is electromagnetic vibrations(waves in an electromagnetic field). However, within the framework we had to return again to corpuscular concepts of light, and at the same time the concept of a photon - a quantum of light - appeared. Since then, it has been believed that light has a dual nature - in some cases it is wave, in others it is corpuscular.
Field physics significantly changes the philosophy of all these issues. Firstly, it separates the concept to which the basic (protons, electrons, etc.) and bodies consisting of them belong, from the concept to which light belongs, as an electromagnetic component. The light is not a material entity, it is oscillatory process, which can be characterized by such concepts as frequency or , but does not have or .
According to this philosophy, light does not obey laws valid for material bodies. In particular, they cannot act on it; the classical rule of addition is not applicable to it, since light is an essence of a different nature than material objects. So if you throw a stone from a moving boat, then its total speed relative to the shore will be the sum initial speed stone and boat speed. If the stone will fall into the water, then the speed of propagation of circles on the water does not depend on the speed at which the stone was flying, since the circles on the water, like light, are nothing more than a material body. The speed of waves is determined by the properties of the medium in which they propagate, and it does not depend on the speed of the source that created these waves (the speed of the source affects the frequency of the waves, this effect is called the Doppler effect). This simple explanation clearly shows why, unlike the speed of the stone, it does not depend on the source. It’s just that the law of addition of velocities, applicable to material bodies, is not applicable to light, as an entity of a different nature.
According to the deflection of light, it is also not associated with the effect of gravitational forces on light, since light, as an oscillatory process, does not possess (or rather, a gravitational charge). This effect occurs due to an increase in the environment near large body and, therefore, light experiences some refraction when passing through a denser medium. Similarly, in field physics, many effects associated with light receive a completely different interpretation and explanation.