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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Critical angle (optics) ： ウィキペディア英語版 Total internal reflection Total internal reflection is a phenomenon which occurs when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. The critical angle is the angle of incidence above which the total internal reflection occurs. This is particularly common as an optical phenomenon, where light waves are involved, but it occurs with many types of waves, such as electromagnetic waves in general or sound waves. When a wave reaches a boundary between different materials with different refractive indices, the wave will in general be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the direction of propagation is closer to being parallel to the boundary) than the critical angle – the angle of incidence at which light is refracted such that it travels along the boundary – then the wave will not cross the boundary, but will instead be totally refracted back internally. This can only occur when the wave in a medium with a higher refractive index (''n''1) reaches a boundary with a medium of lower refractive index (''n''2). For example, it will occur with light reaching air from glass, but not when reaching glass from air. ==Optical description== Total internal reflection of light can be demonstrated using a semi-circular block of glass or plastic. A "ray box" shines a narrow beam of light (a "ray") onto the glass medium. The semi-circular shape ensures that a ray pointing towards the centre of the flat face will hit the curved surface at a right angle; this will prevent refraction at the air/glass boundary of the curved surface. At the glass/air boundary of the flat surface, what happens will depend on the angle. Where θc is the critical angle measurement which is caused by the sun or a light source (measured normal to the surface): * If θ < θc, the ray will split. Some of the ray will reflect off the boundary, and some will refract as it passes through. This is not total internal reflection. * If θ > θc, the entire ray reflects from the boundary. None passes through. This is called total internal reflection. This physical property makes optical fibers useful and prismatic binoculars possible. It is also what gives diamonds their distinctive sparkle, as diamond has an unusually high refractive index. ==Critical angle== ''The critical angle'' is the angle of incidence ''above'' which total internal reflection occurs. The angle of incidence is measured with respect to the normal at the refractive boundary (see diagram illustrating Snell's law). Consider a light ray passing from glass into air. The light emanating from the interface is bent towards the glass. When the incident angle is increased sufficiently, the transmitted angle (in air) reaches 90 degrees. It is at this point no light is transmitted into air. The critical angle $\backslash theta\_c$ is given by Snell's law, :$n\_1\backslash sin\backslash theta\_i\; =\; n\_2\backslash sin\backslash theta\_t\; \backslash quad$. Rearranging Snell's Law, we get incidence :$\backslash sin\; \backslash theta\_i\; =\; \backslash frac\; \backslash sin\; \backslash theta\_t$. To find the critical angle, we find the value for $\backslash theta\_i$ when $\backslash theta\_t\; =$90° and thus $\backslash sin\; \backslash theta\_t\; =\; 1$. The resulting value of $\backslash theta\_i$ is equal to the critical angle $\backslash theta\_c$. Now, we can solve for $\backslash theta\_i$, and we get the equation for the critical angle: :$\backslash theta\_c\; =\; \backslash theta\_i\; =\; \backslash arcsin\; \backslash left(\; \backslash frac\; \backslash right),$ If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence. If for example, visible light were traveling through acrylic glass (with an index of refraction of approximately 1.50) into air (with an index of refraction of 1.00), the calculation would give the critical angle for light from acrylic into air, which is :$\backslash theta\; \_=\backslash arcsin\; \backslash left(\; \backslash frac\; \backslash right)=41.8$ is greater than 1, then arcsine is not defined—meaning that total internal reflection does not occur even at very shallow or grazing incident angles. So the critical angle is only defined when $/$ is less than or equal to 1. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Total internal reflection」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.