Optical fibers are based entirely on this principle of total internal reflection. In that case all the light is totally reflected off the interface, obeying the law of reflection. This gives:įor any angle of incidence larger than the critical angle, Snell's law will not be able to be solved for the angle of refraction, because it will show that the refracted angle has a sine larger than 1, which is not possible. The critical angle can be found from Snell's law, putting in an angle of 90° for the angle of the refracted ray. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection. If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. This has an interesting implication: at some angle, known as the critical angle, light travelling from a medium with higher n to a medium with lower n will be refracted at 90° in other words, refracted along the interface. Conversely, light traveling across an interface from higher n to lower n will bend away from the normal. When light crosses an interface into a medium with a higher index of refraction, the light bends towards the normal. If light is travelling from medium 1 into medium 2, and angles are measured from the normal to the interface, the angle of transmission of the light into the second medium is related to the angle of incidence by Snell's law : The change in speed that occurs when light passes from one medium to another is responsible for the bending of light, or refraction, that takes place at an interface. The frequency, wavelength, and speed are related by: The index of refraction can also be stated in terms of wavelength:Īlthough the speed changes and wavelength changes, the frequency of the light will be constant. When light travels from one medium to another, the speed changes, as does the wavelength. The speed of light in a given material is related to a quantity called the index of refraction, n, which is defined as the ratio of the speed of light in vacuum to the speed of light in the medium: When light travels through something else, such as glass, diamond, or plastic, it travels at a different speed. When we talk about the speed of light, we're usually talking about the speed of light in a vacuum, which is 3.00 x 10 8 m/s. Relevant sections in the book : 26.1 - 26.3 Refraction, Snell's law, and total internal reflection
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