What is light in physics - definition, properties and nature of light

Lighting sources: natural and artificial

By taking the passport data for a classic incandescent lamp and an LED, you can find out the luminous efficiency of each product. To do this, the flux indicated in lumens is divided by the power consumption (W).

Comparison of different light sources

How is illumination measured?

These data clearly demonstrate the benefits of new technological solutions.

It should be emphasized! Modern LED lamps are several times more economical compared to their gas-discharge counterparts. Unlike the latter, they do not contain harmful substances. Potential consumers are attracted by their durability and resistance to mechanical stress.

Sunlight and moonlight are natural sources. The physiological characteristics of a person were formed taking into account the corresponding spectral distribution.

T. Jung's experience

To demonstrate the phenomenon of interference and to determine the wavelength of light, T. Jung performed a special experiment.

To obtain a clear interference pattern, it is necessary to have two coherent light beams, that is, beams that have the same wavelength and have a constant phase difference. By illuminating two close slits, it is possible to obtain conditions for the addition and subtraction of light waves behind them on the screen - interference. The main idea of ​​Young's experiment was to use not a simple light beam to illuminate the slits, which was used by researchers before Young, but a beam passing through a small hole, behind which, in accordance with Huygens' principle, a single coherent wave will be excited, and it is this that should used to illuminate two slits to produce an interference pattern behind them.

Rice. 2. Jung's experience of interference.

Law of rectilinear propagation of light

Any student who has entered grades 9-11 should know that light in a homogeneous medium travels along a straight path, and its speed is 3x10 8
m/s
. At this speed, the beam travels from the Earth to the Moon (the distance between them is 384,000 kilometers) in just about 1.2-1.3 seconds!

Based on the rectilinear propagation of light, many concepts are derived, such as shadow, angle of incidence and reflection, and much more. Different branches of science use these data differently, but they are of great importance in technology and theory.

To summarize, we will say that the lexical meaning of the Greek word “photon” clearly conveys its meaning - this is light. Light is both an electromagnetic wave and a stream of photon particles that propagate from the radiation source and fill the entire surrounding space according to the laws of rectilinear propagation, diffraction, interference, etc.

Both natural and artificial lighting have the same properties, with the exception of the wavelength, its amplitude and other, more specific characteristics of each wave.

Why is the sky blue and the grass green?

Actually, these are two questions, not one. And so we will give two different but related answers. We see a clear sky at midday blue because short wavelengths of light are scattered more efficiently when they collide with gas molecules in the atmosphere than long wavelengths. So the blueness we see in the sky is blue light scattered and reflected many times by atmospheric molecules.

But at sunrise and sunset the sky can take on a reddish color. Yes, this happens, believe me. This happens because when the Sun is close to the horizon, light has to travel a longer distance through a much denser (and dustier) layer of atmosphere to reach us than when the Sun is at its zenith. All short waves are absorbed, and we have to be content with the long ones, which are responsible for the red part of the spectrum.

But with grass everything is slightly different. It appears green because it absorbs all wavelengths except green. She doesn't like green, you see, so she reflects them back into our eyes. For the same reason, any object has its own color - we see that part of the light spectrum that it could not absorb. Black objects appear black because they absorb all wavelengths without reflecting anything, while white objects, on the contrary, reflect the entire visible spectrum of light. This also explains why black heats up much more in the sun than white.

The sky is blue, the grass is green, a dog is man's friend

Energy and momentum of a photon

Each photon carries a certain amount of energy. This quantity is called photon energy.

The photon momentum is related to energy by the following relationship:

We substitute the formula for photon energy instead of E: p = hv/c

And instead of frequency, the formula v = c/λ: p = hc/cλ

We reduce the speed of light and get the momentum formula.

Electromagnetic nature of light

It is a proven scientific fact that an ordinary sunbeam is an electromagnetic wave. Many minds worked on this, in particular, Einstein, Vavilov and others. More than one section of physics is devoted to proving the fact that light arises as a result of various excitations in atoms and molecules.

This may be thermal, chemical or electromagnetic effects. When undergoing various processes in an atom, it emits energy quanta in the entire visible range.

The determination of the electromagnetic nature of radiation has been proven by many experiments, as well as by theory. The most complete description of these issues was given by the famous scientist Maxwell in his equations on electromagnetism.

What parameters are used to measure light intensity?

The average buyer doesn’t even think about how light is measured and how important this information is. After all, light, being a physical quantity, is measured according to many quantitative and qualitative parameters. They must be taken into account when planning renovations in the apartment and counting the number of light bulbs needed for each room.

Light can be measured according to the following characteristics:

• intensity;
• strength;
• brightness

You won’t be able to determine all the necessary parameters just by eye, so you should take care of purchasing devices that will help you maintain your vision and a positive psychological attitude at any time of the day.

Is it really necessary to measure the degree of illumination and what is the unit of measurement of light?

Scientists have proven that dim or, conversely, too bright light damages the retina of the human eye, causing visual acuity to deteriorate. Due to the destruction of the retina, the speed and quality of brain functioning decreases. Insufficient brightness increases drowsiness in people, reduces performance and worsens mood

It should be noted that we do not take into account situations in which a dim glow decorates the setting: a romantic date, watching a movie, and so on. A saturated light flux adds strength, energy, desire to work, thereby tiring a person faster

The unit of measurement of light is established by SanPiN and is called sanitary rules and norms - data that must be followed when measuring illumination. Measurements are taken to determine not only the degree of illumination, but also the level of noise, dust, pollution, and vibration. According to doctors, a constant lack of light in the workplace leads to overwork of employees, deterioration of vision and concentration. Workers become less able to work, which can result in accidents due to inattention or other reasons.

In addition to people, other living organisms also suffer from insufficient lighting: plants, animals. For rapid development and fertile flowering, plants definitely need a powerful stream of light. In animals, due to poor-quality lighting, disturbances in growth and development, reproductive function, weight gain may occur, and the activity of the creature may decrease.

Light quantity measurement for LED devices

For clarity, it is convenient to present the value of illumination in typical situations. These values ​​can be compared with the parameters provided by manufacturers of LED devices in the accompanying documentation.

Illuminance table

In advertising brochures, to improve sales, a light bulb may be called bright and energy-saving. To make the correct conclusion about the consumer parameters of the product, you can use the information presented above.

Wave properties of light

The fact that light is a wave of radiation with certain wave properties began to be assumed by many scientists back in the 17th and 18th centuries. The experiments of Young, Fresnel, and Newton clearly showed that wave characteristics are expressed in two key phenomena: diffraction and interference.

They are the ones that matter when proving that we are dealing with a wave.

A beam of visible radiation is capable of bending around obstacles of any shape and illuminating even an area that is supposedly in the shadow. Deviation from linear propagation, which is impossible for solid particles, is called diffraction.

It has also been proven that radiation can overlap each other and, as it were, complement waves of a similar nature, or “extinguish” or reduce their intensity. This phenomenon is called interference.

It is actively used, for example, in the production of car headlights - their glasses have a special texture that makes it possible to use interference and maximize the intensity of the glow.

But the assertion that light is only a wave also faces protests. Since other experiments, say, of the Russian scientist Vavilov, show that he is characterized by a dual characteristic.

General information

Every person vitally needs light of high quality in all physical indicators. Light output, brightness and intensity of luminous flux both in natural and artificial conditions require indispensable planning, organization and subsequent monitoring of the existing indicators of the listed criteria. Any light source we use is represented by an object emitting it in relation to the surface of the illuminated object. The more light waves that fall on a given surface area, the more clearly and better the object in question is visible. It is this quantity that is called illumination or light intensity in physics.

A specific type of distribution of the intensity of light waves in the space where they directly fall is called interference

The physical properties of the light flux affect the health of the eyes, nervous system and other vital human organs, therefore it is important to approach the arrangement of living, educational and work premises with special care

Light, as a physical factor, has many characteristics studied by specialists, each of which has its own units of measurement. We list the main ones:

• Luminous intensity is a parameter of energy distributed over a certain period of time in a certain direction. The unit of measurement is candelas (Cd).
• Illumination - shows the ratio of the flux of light to the surface on which it falls. The unit of measurement is lux (Lx).
• Luminous output - reflects the relationship between light and the power of the lighting devices that emit it. The unit of measurement is lumens (Lm).

Thus, in optics there are several important physical factors - light output, brightness, strength and intensity, which are important to consider when purchasing specialized lighting devices and accessories for them, subsequent planning and equipping a room with them. For most ordinary buyers, it is quite difficult to understand these nuances.

The most informative are the units of measurement indicated by the manufacturer on the product packaging - Lm/W, CD, Lx, etc. We invite you to find out how light intensity is measured further.

And what is there beyond the visible region of the spectrum?

As the waves get shorter, the color changes from red to blue to violet and finally visible light disappears. But the light itself did not disappear - it moved into the region of the spectrum called ultraviolet. Although we no longer perceive this part of the light spectrum, it is what makes fluorescent lamps, some types of LEDs, and all sorts of cool glow-in-the-dark things glow. Next comes X-ray and gamma radiation, with which it is better not to deal at all.

At the other end of the visible light spectrum, where red ends, infrared radiation begins, which is more heat than light. It could very well fry you. Then comes microwave radiation (very dangerous for eggs), and even further - what we used to call radio waves. Their lengths are already measured in centimeters, meters and even kilometers.

Wavelength of light

By measuring the optical difference in the path of light rays and the distance between the interference fringes in Young's experiment, we can obtain a formula by which the light wavelength is calculated. Another option for determining the length of light is to measure the radius of Newton's rings and calculate the difference in the optical path of the waves based on the geometry of the lens.

It turned out that the light consists of waves of very short wavelengths - from $4×10^{-7}m$ for violet to $8x10^{-7}m$ for red colors.

Rice. 3. Light wavelengths.

The undulating nature of light waves and their length impose restrictions on the maximum magnification of a light microscope. Objects that are smaller than the wavelength of light will be “bent around” by the light waves and will be impossible to see. Therefore, for greater magnification, electron microscopes use a flow of electrons, which, in accordance with the principle of wave-particle duality, have the properties of waves with a very short length.

Color and spectral colors

What is color? Physics gives the following answer to this question: Color is a qualitative subjective characteristic of electromagnetic radiation in the optical range, determined on the basis of the emerging physiological visual sensation, and depending on a number of physical, physiological and psychological factors. [1.1]

Individual perception of color is determined by its spectral composition, as well as color and brightness contrast with surrounding light sources and non-luminous objects. [1.1]

In a continuous light spectrum, in which some colors smoothly transition into others so that it is difficult to determine exactly the boundaries of each color and its relationship with a certain wavelength, it is customary to distinguish the following colors depending on the wavelength [3.1]:

The wave range from 0 nm to 380 nm is considered invisible and is called the ultraviolet region of optical radiation.

The wave range from 780 nm to 1 mm is considered invisible and is called the infrared region of optical radiation.

Continuous optical spectrum

Figure 1 shows the main maximum of the diffraction color spectrum.

The visual organs of living beings perceive light reflected from physical objects and objects. The color of an object perceived by the visual organs will correspond to the wavelengths reflected by these objects. For example, foliage appears green to us because the leaf reflects the green component of the spectrum, and, on the contrary, absorbs all other components. Or another example: an orange is orange, because it is the orange component of the light spectrum that is reflected by the orange.

The sensitivity of the visual organs of living beings is not constant in the zone of the visible light spectrum. For a person, for example, based on data from [3.2], the sensitivity of the visual organs is shown in Figure 2.

The spectral sensitivity of rod vision (Figure 2, curve 2 - the eye is adapted to night brightness) characterizes the operation of the eye with such a small amount of light that it is not enough to even partially excite the cones. The relative spectral sensitivity curve of the eye has a maximum at a wavelength of 507 nm.

For an eye adapted to daytime brightness V(λ) (Figure 2, curve 1), a twofold decrease in sensitivity is characteristic at wavelengths of 510 nm and 610 nm. If the eye is adapted to night brightness V'(λ) (Figure 2, curve 2), then a twofold decrease in sensitivity is observed at wavelengths of 455 nm. and 550 nm.

Figure 2. Relative spectral sensitivity of the human eye

The maxima on curves 1 and 2 in Figure 2, equal to unity, are relative. The fact is that the rod apparatus of human night vision is much more sensitive, and to perceive an extremely small light signal (for example, a barely visible point on a dark background), rods require approximately five hundred times less power than cones. In this case, the rods, which operate in peripheral (lateral) vision, do not allow one to determine the color of a point, while the cones, which fix a point in direct vision, make it possible to see its color [3.3].

In addition, the sensitivity of the human eye to different color components of light varies. Sensitivity is maximum at 555 nm (yellow-green light) and minimizes at longer (red light) and shorter (blue light) wavelengths. The sensitivity of the human eye to red radiation (650 nm) is only 10% of the maximum sensitivity. In other words, to achieve the same brightness sensation as yellow-green light, the intensity of red light must be ten times greater [4.1].

If we combine the visible red and blue ranges of the spectrum, we get a color wheel Figure 3. The color wheel is a way of representing the continuity of color transitions in the visible part of the spectrum. The sectors of the circle are painted in different color tones, placed in the order of spectral colors, with magenta connecting the extreme red and violet colors.

Figure 3. Color wheel and triads of colors that produce white when mixed.

The color wheel was first proposed by Isaac Newton in 1704. The color wheel is of great importance for understanding the laws of mixing spectral colors. So, for example, the vertices of a triangle inscribed in the color wheel clearly indicate triads of colors that, when mixed, will give white.

Figure 4. Visible spectrum color field.

In general, the color shades obtained by mixing simple spectral colors are presented in Figure 4.