Today we will talk about transmittance and related concepts. All these quantities refer to the section of linear optics.

## Light in the ancient world

People used to think the world was full of mysteries. Even the human body carried a lot of the unknown. For example, the ancient Greeks did not understand how the eye sees, why color exists, why night comes. But at the same time, their world was simpler: light, falling on an obstacle, created a shadow. This is all that even the most educated scientist needed to know. No one thought about the transmittance of light and heating. And today they study it at school.

## Light meets obstacle

When a stream of light hits an object, it can behave in four different ways:

• gobble up;
• scatter;
• reflect;
• move on.

Accordingly, any substance has coefficients of absorption, reflection, transmission and scattering.

Absorbed light changes the properties of the material itself in different ways: heats it, changes its electronic structure. Diffused and reflected light are similar, but still different. When reflecting lightchanges the direction of propagation, and when scattered, its wavelength also changes.

## A transparent object that transmits light and its properties

Reflection and transmission coefficients depend on two factors - the characteristics of light and the properties of the object itself. It matters:

1. Aggregate state of matter. Ice refracts differently than steam.
2. The structure of the crystal lattice. This item applies to solids. For example, the transmittance of coal in the visible part of the spectrum tends to zero, but a diamond is a different matter. It is the planes of its reflection and refraction that create a magical play of light and shadow, for which people are willing to pay fabulous money. But both of these substances are carbons. And a diamond will burn in a fire no worse than coal.
3. Temperature of matter. Oddly enough, but at high temperatures, some bodies themselves become a source of light, so they interact with electromagnetic radiation in a slightly different way.
4. The angle of incidence of the light beam on the object.

Also, remember that the light that comes out of an object can be polarized.

## Wavelength and transmission spectrum

As we mentioned above, the transmittance depends on the wavelength of the incident light. A substance that is opaque to yellow and green rays appears transparent to the infrared spectrum. For small particles called "neutrinos" the Earth is also transparent. Therefore, despite the fact that theygenerates the Sun in very large quantities, it is so difficult for scientists to detect them. The probability of a neutrino colliding with matter is vanishingly small.

But most often we are talking about the visible part of the spectrum of electromagnetic radiation. If there are several segments of the scale in the book or task, then the optical transmittance will refer to that part of it that is accessible to the human eye.

## Coefficient formula

Now the reader is prepared enough to see and understand the formula that determines the transmission of a substance. It looks like this: S=F/F0.

So, the transmittance T is the ratio of the radiation flux of a certain wavelength that passed through the body (Ф) to the original radiation flux (Ф0).

The value of T has no dimension, as it is denoted as a division of identical concepts into each other. However, this coefficient is not devoid of physical meaning. It shows how much electromagnetic radiation a given substance passes through.

This is not just a phrase, but a specific term. The radiation flux is the power that electromagnetic radiation carries through a unit surface. In more detail, this value is calculated as the energy that radiation moves through a unit area in a unit time. Area is most often a square meter, and time is seconds. But depending on the specific task, these conditions can be changed. For example, for redgiant, which is a thousand times larger than our Sun, you can safely use square kilometers. And for a tiny firefly, square millimeters.

Of course, in order to be able to compare, unified measurement systems were introduced. But any value can be reduced to them, unless, of course, you mess up with the number of zeros.

Associated with these concepts is also the magnitude of the directional transmittance. It determines how much and what kind of light passes through the glass. This concept is not found in physics textbooks. It is hidden in the specifications and rules of window manufacturers.

## The law of conservation of energy

This law is the reason why the existence of a perpetual motion machine and a philosopher's stone is impossible. But there are water and windmills. The law says that energy does not come from nowhere and does not dissolve without a trace. Light falling on an obstacle is no exception. It does not follow from the physical meaning of the transmittance that since part of the light did not pass through the material, it evaporated. In fact, the incident beam is equal to the sum of the absorbed, scattered, reflected, and transmitted light. Thus, the sum of these coefficients for a given substance should be equal to one.

In general, the law of conservation of energy can be applied to all areas of physics. In school problems, it often happens that the rope does not stretch, the pin does not heat up, and there is no friction in the system. But in reality this is impossible. In addition, it is always worth remembering that people knowNot all. For example, in beta decay, some of the energy was lost. Scientists did not understand where it went. Niels Bohr himself suggested that the conservation law might not hold at this level.

But then a very small and cunning elementary particle was discovered - the neutrino lepton. And everything fell into place. So if the reader, when solving a problem, does not understand where the energy goes, then we must remember: sometimes the answer is simply unknown.

## Application of the laws of transmission and refraction of light

A little higher we said that all these coefficients depend on what substance gets in the way of the electromagnetic radiation beam. But this fact can also be used in reverse. Taking the transmission spectrum is one of the simplest and most effective ways to find out the properties of a substance. Why is this method so good?

It is less accurate than other optical methods. Much more can be learned by making a substance emit light. But this is the main advantage of the optical transmission method - no one needs to be forced to do anything. The substance does not need to be heated, burned or irradiated with a laser. Complex systems of optical lenses and prisms are not required as the beam of light passes directly through the sample under study.

In addition, this method is non-invasive and non-destructive. The sample remains in its original form and condition. This is important when the substance is scarce, or when it is unique. We are sure that Tutankhamun's ring is not worth burning,to find out more precisely the composition of the enamel on it.