Introduction: Bending the Light: the Magic of Refraction of Light in Different Mediums

Refraction of light is the bending of light as it passes from one medium to another with a different optical density. Refraction is responsible for various optical phenomena, including the bending of light in lenses, the formation of rainbows, and the apparent bending of objects in water.

Exploring the Path of Light: Refraction of light  in Various Mediums

• 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: 
    • 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. 

Refraction of Light in Action: The Experiment with a Rectangular Glass Slab

• 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.

Guiding Principles: Understanding the Laws of Refraction of Light 

• (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.

Exploring Refraction of Light through the Lens of Refractive Index

• 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. 

Conclusion
In summary, the exploration of refraction of light reveals its influence on various optical phenomena. From lenses to rainbows, this phenomenon shapes our visual experiences. Laws like Snell’s Law and the refractive index provide a mathematical framework for understanding light’s behavior. Altogether, the study of refraction enriches our insight into the dynamic interplay between light and different mediums.

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