Neighbourhood market 180-200 words article
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Explanation:
tal internal reflection (TIR) is the phenomenon that makes the water-to-air surface in a fish-tank look like a perfectly silvered mirror when viewed from below the water level (Fig. 1). Technically, TIR is the total reflection of a wave incident at a sufficiently oblique angle on the interface between two media, of which the second ("external") medium is transparent to such waves but has a higher wave velocity than the first ("internal") medium. TIR occurs not only with electromagnetic waves such as light waves and microwaves, but also with other types of waves, including sound and water waves. In the case of a narrow train of waves, such as a laser beam, we tend to speak of the total internal reflection of a "ray" (Fig. 2).
Fig. 2: Repeated total internal reflection of a 405 nm laser beam between the front and back surfaces of a glass pane. The color of the laser light itself is deep violet; but its wavelength is short enough to cause fluorescence in the glass, which re-radiates greenish light in all directions, rendering the zigzag beam visible.
Refraction is generally accompanied by partial reflection. When a wavetrain is refracted from a medium of lower propagation speed (higher refractive index) to a medium of higher propagation speed (lower refractive index), the angle of refraction (between the refracted ray and the normal to the refracting interface) is greater than the angle of incidence (between the incident ray and the normal to the interface). Hence, as the angle of incidence approaches a certain limit, called the critical angle, the angle of refraction approaches 90°, at which the refracted ray becomes tangential to the interface. As the angle of incidence increases beyond the critical angle, the conditions of refraction can no longer be satisfied; so we have no refracted ray, and the partial reflection becomes total. In an isotropic medium such as air, water, or glass, the ray direction is simply the direction normal to the wavefront.
If the internal and external media are isotropic with refractive indices n1 and n2 respectively, the critical angle is given by {\displaystyle \theta _{{\text{c}}\!}=\arcsin(n_{2}/n_{1})} {\displaystyle \theta _{{\text{c}}\!}=\arcsin(n_{2}/n_{1})}, and is defined if n2 ≤ n1. For example, for visible light, the critical angle is about 49° for incidence from water to air, and about 42° for incidence from common glass to air.
Details of the mechanism of TIR give rise to more subtle phenomena. While total reflection, by definition, involves no continuing transfer of power across the interface, the external medium carries a so-called evanescent wave, which travels along the interface with an amplitude that falls off exponentially with distance from the interface. The "total" reflection is indeed total if the external medium is lossless (perfectly transparent), continuous, and of infinite extent, but can be conspicuously less than total if the evanescent wave is absorbed by a lossy external medium ("attenuated total reflectance"), or diverted by the outer boundary of the external medium or by objects embedded in that medium ("frustrated" TIR). Unlike partial reflection between transparent media, total internal reflection is accompanied by a non-trivial phase shift (not just zero or 180°) for each component of polarization (normal or parallel to the plane of incidence), and the shifts vary with the angle of incidence. The explanation of this effect by Augustin-Jean Fresnel, in 1823, added to the evidence in favor of the wave theory of light.
The phase shifts are utilized by Fresnel's invention, the Fresnel rhomb, to modify polarization. The efficiency of the reflection is exploited by optical fibers (used in telecommunications cables and in image-forming fiberscopes), and by reflective prisms, such as erecting prisms for binoculars