# What is the unit for measuring the intensity of light? What is the intensity of light measured in?

Education

Today we will talk about the unit of measurement of the intensity of light. This article will reveal to readers the properties of photons that will help determine why light is of different brightness.

## A particle or a wave?

At the beginning of the twentieth century, scientists puzzledthe behavior of quanta of light-photons. On the one hand, interference and diffraction spoke about their wave essence. Consequently, light characterized such properties as frequency, wavelength and amplitude. On the other hand, Lebedev's experiments convinced the scientific community that photons transmit momentum to the surfaces. It would be impossible, do not have a particle of mass. Thus, physicists had to admit: electromagnetic radiation is both a wave and a material object.

## Photon energy

As Einstein proved, mass is energy.This fact proves our central star, the Sun. The thermonuclear reaction converts the mass of a highly compressed gas into pure energy. But how to determine the power of the emitted radiation? Why in the morning, for example, the sun's light intensity is lower than at noon? The characteristics described in the previous section are related to each other by concrete relationships. And all of them point to the energy carried by electromagnetic radiation. This value changes to the larger side when:

• decrease in wavelength;
• increase in frequency.

## What is the energy of electromagnetic radiation?

The photon is different from the rest of the particles.Its mass and, consequently, energy exist only as it moves through space. When a collision with an obstacle a quantum of light increases its internal energy or gives it a kinetic moment. But the photon itself ceases to exist. Depending on what exactly is an obstacle, there are various changes.

1. If the obstacle is a solid, then most oftenlight heats it. The following scenarios are also possible: the photon changes the direction of motion, stimulates a chemical reaction, or causes one of the electrons to leave their orbit and go to a different state (photoelectric effect).
2. If the obstacle is the only molecule, for example, from a rarefied cloud of gas in open space, then the photon causes all its connections to oscillate more strongly.
3. If the obstacle is a massive body (for example, a star or even a galaxy), the light distorts and changes the direction of motion. This effect is based on the ability to "look" into the distant past of the cosmos.

## Science and humanity

Scientific data often seems to be something abstract,inapplicable to life. This happens with the characteristics of light. If we are talking about an experiment or measuring the emission of stars, scientists need to know the absolute values ​​(they are called photometric). These concepts, as a rule, are expressed in terms of energy and power. Recall, power means the rate of change in energy per unit time, and in general it shows the amount of work that the system can produce. But man is limited in his ability to experience reality. For example, the skin feels warm, but the eye does not see the photon of infrared radiation. The same problem also applies to units of light intensity: the power that the radiation demonstrates is actually different from the power that the human eye can perceive.

## Spectral sensitivity of the human eye

We remind you that we will discuss below the averagedindicators. All people are different. Some do not perceive the individual colors (color blinds) at all. For others, the culture of color does not coincide with the generally accepted scientific point of view. For example, the Japanese do not distinguish between green and blue, and the British - blue and blue. In these languages, different colors are denoted by one word.

The unit of luminous intensity depends on the spectralsensitivity of the average human eye. The maximum of daylight falls on a photon with a wavelength of 555 nanometers. This means that in the sunlight the person sees the green color best. The maximum of night vision is a photon with a wavelength of 507 nanometers. Therefore, when the Moon is the people better see the blue objects. In the twilight, everything depends on the lighting: the better it is, the more "green" becomes the maximum color that a person perceives.

## The structure of the human eye

Almost always, when it comes to seeing, wewe say that he sees the eye. This is an incorrect statement, because the brain first of all perceives. The eye is only a tool that transmits information about the light flux to the host computer. And, like any instrument, the whole system of perception of colors has its limitations.

There are two different types in the human retinacells - cones and rods. The first are responsible for day vision and better perceive colors. The second provide night vision, thanks to the chopsticks the person distinguishes light and shadow. But they do not take color well. Sticks are also more sensitive to movement. That's why, if a person walks through the moonlit park or forest, he notices every wiggle of branches, every sigh of the wind.

The evolutionary reason for this separation is simple:we have one sun. The moon shines with reflected light, which means that its spectrum does not differ much from the spectrum of the central luminary. Therefore, the day is divided into two parts - lighted and dark. If people lived in a system of two or three stars, then our vision might have more components, each of which was adapted to the spectrum of one luminary.

I must say, there are beings on our planet whosevision is different from the human. Deserted inhabitants, for example, with their eyes catch infrared light. Some fish see the near ultraviolet, since this radiation penetrates the depth of the water more deeply. Our pets, cats and dogs, perceive colors differently, and their spectrum is cut: they are better adapted to chiaroscuro.

But people are all different, as we mentioned above.Some representatives of mankind see near infrared light. We can not say that they would not need thermal imagers, but they are able to perceive slightly more red shades than most. In others, the ultraviolet part of the spectrum is developed. Such a case is described, for example, in the film "The Planet Ka-Peaks". The protagonist claims that he came from another star system. The examination revealed in him the ability to see ultraviolet radiation.

Does this prove that Prot is an alien? No.Some people can do it. In addition, the near ultraviolet closely adjoins the visible spectrum. It's no wonder that someone perceives a little more. But Superman is definitely not from the Earth: the x-ray spectrum is too far from the visible, so that such a vision could be explained from the human point of view.

## Absolute and relative units for determining the luminous flux

Independent of spectral sensitivityThe quantity that shows the flow of light in a known direction is called "candela". The unit of power is already pronounced with a more "human" attitude. The only difference is in the mathematical notation of these concepts: the absolute value has a lower index "e", relative to the human eye - "υ". But do not forget that the values ​​of these categories will be very different. This must be taken into account when solving real problems.

## Enumeration and comparison of absolute and relative values

To understand what is measured by the power of light,it is necessary to compare "absolute" and "human" values. On the right are purely physical concepts. On the left are the magnitudes into which they transform when passing through the system of the human eye.

1. The power of radiation becomes the power of light. Concepts are measured in candela.
2. Energy brightness turns into brightness. Values ​​are expressed in candelas per square meter.

Surely the reader saw familiar words here.Many times in their lives people say: "Very bright sun, go into the shadows" or "Make the monitor brighter, the film is too gloomy and dark." We hope that the article will slightly clarify where this concept came from, and also how the unit of luminous intensity is called.

## Features of the concept of "candela"

A little higher, we have already mentioned this term.We also explained why the same word refers to completely different concepts of physics related to the power of electromagnetic radiation. So, the unit of measurement of the power of light is called "candela". But what is it equal to? One candela is the force of light in a known direction from a source that emits strictly monochromatic radiation with a frequency of 5.4 * 1014, and the energy source strength in thisdirection is equal to 1/683 Watt per unit solid angle. To translate the frequency into a wavelength, the reader can quite self, the formula is very easy. Let's tell: the result lies in the visible area.

The unit of measurement of the intensity of light is called"Candela" for good reason. Those who know English, remember that candle is a candle. Previously, many areas of human activity were measured in natural parameters, for example, horsepower, millimeters of mercury. So it's no wonder that the unit of measurement of the strength of light is candela, one candle. Only a candle is very peculiar: with a strictly specified wavelength, and producing a specific number of photons per second.