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Refraction
Refraction
Figure 1: Light Reflecting through a plastic block
As shown in Figure 1, refraction is the redirection (or bending) of a wave as it passes from one medium (or substance) to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction. How much a wave is refracted is determined by the change in wave speed and the initial direction of wave propagation relative to the direction of change in speed.
Refractive Index
Refractive Index
The refractive index, nD, is the ratio of the speed of light in a vacuum c, divided by the speed of light in a particular material, v. The "D" comes about because the refractive index is usually referenced to a specific wavelength, the sodium "D" line which occurs at 589 nm. The speed of light in a vacuum is 299,792,458 m/s, but we usually use 3.00 x 108 m/s.
The refractive index is also called the index of refraction.
Refraction of Light at an Interface
Refraction of Light at an Interface
Figure 2: Light travels from material 1 at the angle of incidence 𝛂 into material 2 at the angle of refraction 𝛽.
When light travels from one material to another, the light refracts (or bends) such that there is a relationship between the angle of incidence, the angle of refraction and the refractive indices of the two materials. In the picture to the left, substance 2 has a higher refractive index than substance 1, so the angle of refraction is smaller than the angle of incidence.
Snell's Law
Snell's Law
An image of light traveling from one material to another material is shown in Figure 2. Light is represented as the traveling arrow. The light grey vertical line is a line perpendicular to the boundary between materials 1 and 2. 𝛼 and 𝛽 are the angles between the path of the light and the line perpendicular to the boundary.
Refraction of light like in the figure above is governed by Snell’s Law,
where nD1 is the refractive index of material 1, nD2 is the refractive index of material 2, 𝛼 is the angle of incidence, and 𝛽 is the angle of refraction. Snell’s Law can be rearranged to solve for the refractive index of material 2,
This requires you to use your “sin” button on your calculator. Make sure your calculator is in degrees mode and displays “DEG” on the screen. If not, ask your instructor about how to do this calculation. Many students now have graphing calculators that don’t allow you to input angles in degrees.
Please continue on to the Refractometer page for more information
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