Studying the laws of motion of light in various transparent media, the concept of refractive index is often used. What is the meaning of a physical quantity, and also for what phenomena it is important, is considered in the article.
Refraction of light
When a ray of light (in fact of any wave) passes through a surface bounding two transparent media, then its rectilinear trajectory is refracted on this surface. The result of this phenomenon is a distortion of the image of objects, if they are in one environment, and look at them from another environment. For example, a clear break is visible if the pencil is placed in a glass of water.
The mathematical law for the phenomenon of refraction was first formulated by the Dutch scientist Snell in the early 1600s. In fairness, we note that many scientists were engaged in refraction, starting with the Greek philosopher Ptolemy and ending with Newton and Descartes. The corresponding formula is:
n 1 * sin (θ 1 ) = n 2 * sin (θ 2 ).
Here θ 1 and θ 2 are the angles between the incident and refracted rays and the normal drawn to the surface at the point of intersection of the light beam. The symbols n 1 and n 2 in the formula indicate the refractive indices of the corresponding transparent materials. What is the meaning of the physical refractive index of the medium, we consider in the next paragraph.
Refractive index (absolute)
In physics, this quantity is introduced as the ratio of two speeds of light in different materials or in a vacuum. It is known that in airless space the speed of light exceeds that for any other material. Therefore, she was chosen for the standard. Denoting the speed of electromagnetic waves in a certain medium as v, we can write the following mathematical definition of the refractive index:
n = c / v.
What is the meaning of the physical refractive index of light in a medium is evident from this formula. The value of n shows how much faster the light moves in airless space than in a given environment.
From the formula it is also clear that n is always equal to one or more. It is equal to unity for vacuum, and is also close to unity for discharged gases. For example, for air n = 1,00029.
Refractive index (relative)
Besides the value of n introduced above, there is also a relative refractive index. Apply it less often in physical calculations than absolute.
Using the formula for absolute n, Snell's law for refraction can be written in the following form:
sin (θ 1 ) / v 1 = sin (θ 2 ) / v 2 =>
sin (θ 1 ) / sin (θ 2 ) = v 1 / v 2 = n 12 .
The value of n 12 is called the relative refractive index for the considered media.
What is the meaning of the physical refractive index n 12 ? This value shows how many times the light in the first medium is faster than in the second. In contrast to the absolute indicator, the relative can be either more than one or less than it.
The knowledge of the refractive index is important for describing the phenomenon of total reflection, which occurs only in an optically denser medium, that is, in a medium with large n. This phenomenon is used in optical fibers.
It is also important to know the refractive index in the manufacture of optical glasses (lenses) for microscopes, telescopes, glasses and other devices.