Absolute refractive index
This article reveals the essence of the concept of optics, as the index of refraction. The formulas for obtaining this quantity are given, a brief overview of the application of the phenomenon of refraction is given.
Ability to see and index of refraction
At the dawn of the birth of civilization, people asked the question: how does the eye see? It has been suggested that a person emits rays that touch surrounding objects, or, conversely, all things emit such rays. The answer to this question was given in the seventeenth century. It is contained in optics and is related to what the refractive index is. Reflecting from various opaque surfaces and refracting on the border with transparent ones, the light gives a person the opportunity to see.
Light and refractive index
Our planet is shrouded in the light of the sun. And it is precisely with the wave nature of photons that such an absolute refractive index is associated. Spreading in vacuum, the photon meets no obstacles. On the planet, light meets many different more dense environments: the atmosphere (mixture of gases), water,crystals. Being an electromagnetic wave, the photons of light have one phase velocity in vacuum (denoted byc), and in the environment - another (indicated byv). The ratio of the first and second is what is called the absolute index of refraction. The formula looks like this: n = c / v.
It is necessary to give a definition of the phase velocity of the electromagnetic environment. Otherwise, understand what the refractive indexn, it is impossible. The photon of light is a wave. So, it can be represented as a packet of energy that oscillates (imagine a segment of a sine wave). A phase is that segment of a sine wave that a wave passes at a given point in time (we recall that this is important for understanding such a value as the refractive index).
For example, a phase may be a maximum of a sinusoid or some segment of its slope. The phase velocity of a wave is the speed with which this particular phase moves. As the definition of the refractive index explains, for a vacuum and for a medium, these values differ. Moreover, each environment has its own value of this magnitude. Any transparent compound, whatever its composition, has a refractive index that is different from all other substances.
Absolute and relative refractive index
It has already been shown above that the absolute value is measured relative to vacuum. However, with this on our planet tight: the light often falls on the boundary of air and water or quartz and spinel. For each of these media, as already mentioned above, its refractive index is different. In air, a photon of light travels along one direction and has one phase velocity (v1), but, falling into the water, changes the direction of propagation and phase velocity (v2). However, both of these directions lie in the same plane. It is very important for understanding how the image of the surrounding world is formed on the retina of the eye or on the matrix of the camera. The ratio of the two absolute values gives the relative refractive index. The formula looks like this: n12= v1/ v2.
But what if the light, on the contrary, comes out of the water and enters the air? Then this value will be determined by the formula n21= v2/ v1. By multiplying the relative refractive indices we get n21* n12= (v2* v1) / (v1* v2) = 1. This ratio is valid for any pair of media. The relative refractive index can be found from the sines of the angles of incidence and refraction n12= sin Ɵ1/ sin Ɵ2. Do not forget that the angles are counted from the normal to the surface. By normal is meant a line perpendicular to the surface. That is, if an angle is given in the problemαfalling relative to the surface itself, it is necessary to consider the sine of (90 - α).
The beauty of the refractive index and its application
On a calm sunny day, glare is played on the bottom of the lake. Dark blue ice covers the rock. On a woman's hand, a diamond scatters thousands of sparks. These phenomena are due to the fact that all the boundaries of transparent media have a relative refractive index. In addition to aesthetic pleasure, this phenomenon can be used for practical use.
Here are some examples:
- A glass lens collects a beam of sunlight and sets fire to the grass.
- The laser beam focuses on the diseased organ and cuts off unwanted tissue.
- Sunlight is refracted on an ancient glass window, creating a special atmosphere.
- Microscope magnifies images of very small parts.
- The lenses of the spectrophotometer collect the laser light reflected from the surface of the studied substance. Thus, it is possible to understand the structure, and then the properties of new materials.
- There is even a photon computer project where information will be transmitted not by electrons, as now, but photons. For such a device will definitely need refractive elements.
Wavelength and refractive index
However, the Sun supplies us with photons not only of the visible spectrum. Infrared, ultraviolet, X-rays are not perceived by human vision, but affect our lives. The infrared rays warm us, UV photons ionize the upper atmosphere and allow plants to produce oxygen through photosynthesis.
And what the refractive index is equal depends not only on the substances between which the boundary lies, but also on the wavelength of the incident radiation. What kind of value in question is usually clear from the context. That is, if a book examines x-rays and its effect on a person, thennthere it is determined for this range. But it usually means the visible spectrum of electromagnetic waves, unless otherwise indicated.
Refractive index and reflection
As it became clear from the above, we are talking about transparent environments. As examples we gave air, water, and diamond.But what about wood, granite, plastic? Is there such a thing as refractive index for them? The answer is complicated, but in general - yes.
First of all, you should take into account exactly what kind of light we are dealing with. Those environments that are opaque to visible photons are cut through with x-rays or gamma rays. That is, if we were all supermen, then the whole world around us would be transparent, but in varying degrees. For example, walls made of concrete would be no denser than jelly, and metal fittings would look like pieces of denser fruit.
For other elementary particles, muons, our planet is generally transparent through and through. At one time, scientists had a lot of trouble proving the very fact of their existence. Muons millions pierce us every second, but the probability of a collision of at least one particle with matter is very small, and it is very difficult to fix it. By the way, soon Baikal will become a place for "catching" muons. Its deep and clear water is ideal for this - especially in winter. The main thing that the sensors are not frozen. Thus, the refractive index of concrete, for example, for X-ray photons makes sense.Moreover, the irradiation of a substance by X-ray is one of the most accurate and important ways to study the structure of crystals.
It is also worth remembering that in a mathematical sense, substances that are opaque for a given range possess an imaginary refractive index. Finally, we must understand that the temperature of a substance can also affect its transparency.