Refraction, physics 10, Geometrical Optics

Physics 10.VI Geometrical Optics T.Trường 9/6/17 1,058 0
  1. Refraction, Geometrical Optics, physics 10
    1/ Refraction

    In the previous sections we studied light reflecting off various surfaces. What happens when light passes through a medium? Like all waves, the speed of light is dependent on the medium in which it is travelling. When light moves from one medium into another (for example, from air to glass), the speed of light changes. The effect is that the light ray passing into a new medium is refracted, or bent. Refraction is therefore the bending of light as it moves from one optical medium to another.

    Definition: Refraction
    Refraction is the bending of light that occurs because light travels at different speeds in different materials.
    When light travels from one medium to another, it will be bent away from its original path.
    When it travels from an optically dense medium like water or glass to a less dense medium like air, it will be refracted away from the normal (Figure 7.9). Whereas, if it travels from a less dense medium to a denser one, it will be refracted towards the normal (Figure 7.10).

    Just as we defined an angle of reflection in the previous section, we can similarly define an angle of refraction as the angle between the surface normal and the refracted ray. This is shown in Figure 7.11.
    [​IMG]
    Figure 7.9: Light is moving from an optically dense medium to an optically less dense medium. Light is refracted away from the normal.
    [​IMG]
    Figure 7.10: Light is moving from an optically less dense medium to an optically denser medium. Light is refracted towards the normal.
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    Figure 7.11: Light moving from one medium to another bends towards or away from the surface normal. The angle of refraction θ is shown.
    2/ Refractive Index
    Which is easier to travel through, air or water? People usually travel faster through air. So does light! The speed of light and therefore the degree of bending of the light depends on the refractive index of material through which the light passes. The refractive index (symbol n) is the ratio of the speed of light in a vacuum to its speed in the material. You can think of the refractive index as a measure of how difficult it is for light to get through a material.
    Definition: Refractive Index
    The refractive index of a material is the ratio of the speed of light in a vacuum to its speed in the medium.
    The symbol c is used to represent the speed of light in a vacuum. c = 299 792 485 m · s$^{−1}$ For purposes of calculation, we use 3 × 108m · s$^{−1}$. A vacuum is a region with no matter in it, not even air. However, the speed of light in air is very close to that in a vacuum.
    Definition: Refractive Index
    The refractive index (symbol n) of a material is the ratio of the speed of light in a vacuum to its speed in the material and gives an indication of how difficult it is for light to get through the material.
    \[n = \dfrac{c}{v}\]​
    where
    • n = refractive index (no unit)
    • c = speed of light in a vacuum (3,00 × 108m · s$^{−1}$)
    • v = speed of light in a given medium ( m · s$^{−1}$)
    Table 7.4.1 shows refractive indices for various materials. Light travels slower in any material than it does in a vacuum, so all values for n are greater than 1.

    3/ Snell’s Law
    Now that we know that the degree of bending, or the angle of refraction, is dependent on the refractive index of a medium, how do we calculate the angle of refraction? The angles of incidence and refraction when light travels from one medium to another can be calculated using Snell’s Law.
    [​IMG]
    Table 7.1: Refractive indices of some materials. n$_{air}$ is calculated at STP and all values are determined for yellow sodium light which has a wavelength of 589,3 nm.
    Definition: Snell’s Law
    n1sin θ1 = n2sin θ2
    where
    • n$_{1 }$= Refractive index of material 1
    • n$_{2 }$= Refractive index of material 2
    • θ1 = Angle of incidence
    • θ2 = Angle of refraction
    Remember that angles of incidence and refraction are measured from the normal, which is an imaginary line perpendicular to the surface. Suppose we have two media with refractive indices n1 and n2. A light ray is incident on the surface between these materials with an angle of incidence θ1. The refracted ray that passes through the second medium will have an angle of refraction θ2.

    High School Students Studying the Sciences Physics
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