Today we’ll talk about transmittance and related concepts. All these quantities are related to linear optics.
Light in the ancient world
Previously, people believed that the world was filled with riddles. Even the human body carried a lot of the unknown. For example, the ancient Greeks did not understand how the eye sees why there is color, why night falls. But at the same time, their world was simpler: light, falling on an obstacle, created a shadow. That was 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 an obstacle
When a stream of light hits an object, it can behave in four different ways:
- absorb
- dissipate
- to reflect;
- pass on.
Accordingly, any substance has transmittance and scattering reflection coefficients.
The absorbed light in various ways changes the properties of the material itself: heats it, changes its electronic structure. Diffused and reflected light are similar, but still different. When reflected, light changes 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 - on the characteristics of light and the properties of the object itself. It matters:
- The aggregate state of the substance. Ice refracts differently than steam.
- 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 another matter. It is the planes of its reflection and refraction that create the magical play of light and shadow, for which people are willing to pay fabulous money. But both of these substances are carbons. And the diamond will burn in the fire no worse than coal.
- The temperature of the substance. Oddly enough, but at high temperature, some bodies themselves become a light source, so they interact with electromagnetic radiation in a slightly different way.
- The angle of incidence of the beam of light on the object.
In addition, we must remember that the light that comes out of the 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 transparent. Therefore, despite the fact that they are generated by the Sun in very large quantities, it is so difficult for scientists to detect them. The probability of a collision of neutrinos with matter is vanishingly small.
But most often we are talking about the visible part of the spectrum of electromagnetic radiation. If in the book or task there are several segments of the scale, the optical transmittance will refer to that part of it that is accessible to the human eye.
Coefficient formula
Now the reader is already prepared enough to see and understand the formula that determines the transmission of a substance. It looks like this: T = f / f 0 .
So, the transmittance T is the ratio of the radiation flux of a certain wavelength that passed through the body () to the initial radiation flux ( 0 ).
The quantity T has no dimension, since it is denoted as dividing the same concepts into each other. However, this coefficient is not without physical meaning. It shows what proportion of electromagnetic radiation a given substance passes.
"The flow of radiation"
This is not just a phrase, but a specific term. A radiation flux is the power that electromagnetic radiation carries through a surface unit. In more detail, this quantity is calculated as the energy that radiation moves through a unit area in a unit time. Under the area most often means a square meter, and under the time - seconds. But depending on the specific task, these conditions can be changed. For example, for the red giant, which is a thousand times larger than our Sun, you can safely apply square kilometers. And for a tiny firefly - square millimeters.
Of course, in order to be able to compare, uniform measurement systems were introduced. But any value can be brought to them, unless, of course, you mess 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.
Law of energy conservation
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 is not taken from anywhere and does not dissolve without a trace. Light incident on an obstacle is no exception. It does not follow from the physical meaning of the transmittance that since a 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 unity.
In general, the law of conservation of energy can be applied to all areas of physics. In school tasks 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 do not know everything. For example, in beta decay, some of the energy was lost. Scientists did not understand where she was going. Niels Bohr himself suggested that the conservation law may not be respected 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
We said a little above that all these coefficients depend on what substance gets in the way of the beam of electromagnetic radiation. But this fact can be used in the opposite direction. 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. You can learn much more if you make a substance emit light. But this is the main advantage of the optical transmission method - no one needs to be forced to 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, since a 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 small, or when it is unique. We are sure that the ring of Tutankhamun should not be burned to find out more precisely the composition of the enamel on it.