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Exploring the Path of Light: Refraction of light in Various Mediums
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- Transparent Medium: Light seems to travel along straight-line paths in a transparent medium.
- But it is also observed that the bottom of a tank or a pond containing water appears to be raised.
- Glass Slab and Refraction of Light: Similarly, when a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab.
- Examples:
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- A pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air and water.
- This is because the light reaching us from the portion of the pencil inside water seems to come from a different direction, compared to the part above water.
- This makes the pencil appear to be displaced at the interface.
- Letters: For similar reasons, the letters appear to be raised, when seen through a glass slab placed over it.
- These observations indicate that, due to the phenomenon of refraction of light, light does not travel in the same direction in all media.
- It appears that when traveling obliquely from one medium to another, the direction of propagation of light in the second medium changes.
- This phenomenon is known as refraction of light.
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Refraction of Light in Action: The Experiment with a Rectangular Glass Slab
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- Experiment: The rectangular glass slab experiment demonstrates that when light, experiencing the phenomenon of refraction of light, passes from one medium (e.g., air) to another medium with a different refractive index (e.g., glass), it undergoes refraction.
- As the incident ray enters the glass slab, undergoing the process of refraction of light, it bends towards the normal line drawn at the point of entry.
- This is because glass has a higher refractive index compared to air.
[Refraction of light through a rectangular glass slab]
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Guiding Principles: Understanding the Laws of Refraction of Light
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- (i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
- (ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the given pair of media.
- This law is also known as Snell’s Law of Refraction of light.
- (This is true for angle 0 < i < 90o ) If i is the angle of incidence and r is the angle of refraction, then, sin i/sin r = constant.
- This constant value is called the refractive index of the second medium with respect to the first.
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Exploring Refraction of Light through the Lens of Refractive Index
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- A ray of light that travels obliquely from one transparent medium into another will change its direction in the second medium.
- The extent of the change in direction that takes place in a given pair of media may be expressed in terms of the refractive index.
- The refractive index, intertwined with the phenomenon of refraction of light, can be linked to an important physical quantity, the relative speed of propagation of light in different media.
- Light propagates with different speeds in different media. Light travels fastest in vacuum with speed of 3×108 m s-1.
- In air, the speed of light is only marginally less, compared to that in vacuum.
- It reduces considerably in glass or water.
- The value of the refractive index for a given pair of media depends upon the speed of light in the two media, as given below.
- Mathematical Expression: Consider a ray of light traveling from medium 1 into medium 2, undergoing the phenomenon of refraction of light.
- Let v1 be the speed of light in medium 1 and v2 be the speed of light in medium 2.
- The refractive index of medium 2 with respect to medium 1 is given by the ratio of the speed of light in medium 1 and the speed of light in medium 2.
- This is usually represented by the symbol n21. It is represented as;
- By the same argument, the refractive index of medium 1 with respect to medium 2 is represented as n12. It is given by;
- If medium 1 is vacuum or air, then the refractive index of medium 2 is considered with respect to vacuum.
- This is called the absolute refractive index of the medium. It is simply represented as n2.
- If c is the speed of light in air and v is the speed of light in the medium, then, the refractive index of the medium nm is given by
- The absolute refractive index of a medium is simply called its refractive index.
- From the table you can know that the refractive index of water, nw = 1.33.
- This means that the ratio of the speed of light in air and the speed of light in water is equal to 1.33.
- Similarly, the refractive index of crown glass, ng = 1.52.
- An optically denser medium may not possess greater mass density.
- Example: Kerosene having higher refractive index, is optically denser than water, although its mass density is less than water.
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[Absolute refractive index of some material media]