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Refractive Index
 


 

When light rays travel from air into another medium, they bend to some extent. Light bends to a greater extent in certain media when compared to others. The refractive index of a medium gives an indication of the light-bending ability of that medium. For example, the refractive index of glass is greater than the refractive index of water. Therefore, light rays bend more on passing from air into glass than from air into water. The refractive index is represented by the letter n. The refractive index is measured in terms of the ratio of speeds of light in the two media. Thus, the ratio of speed of light in vacuum to the speed of light in a medium is called the refractive index of that medium. That is,

Refractive index =
(Of a medium)

Since the speed of light in air is almost equal to the speed of light in vacuum, for all practical purposes. We can say that the ratio of speed of light in air to the speed of light in a medium is called refractive index of that medium. That is,


Refractive index = 
(Of a medium)

Consider a ray of light traveling from medium 1 into medium 2 as shown in the figure. 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 can be represented by n21.


n21= ------------(10.5)

With the same token, one can argue that the refractive index of medium 1 with respect to medium 2 is represented as n12.


n12 = --------------(10.6)

If medium 1 is vacuum or air, the refractive index of medium 2 is considered with respect to vacuum. This is called as 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

nm =  ----------(10.7)


Table 10.3 Absolute refractive index of some Material media

 

Material Medium

Refractive Index

Air

1.0003

Ice

1.31

Water

1.33

Alcohol

1.36

Kerosene

1.44

Fused Quartz

1.46

Turpentine Oil

1.47

Benzene

1.50

Crown glass

1.52

Canada Balsam

1.53

Rock Salt

1.54

Carbon disulphide

1.63

Dense Flint glass

1.65

Ruby

1.71

Sapphire

1.77

Diamond

2.42


Optically Denser Medium – What does it mean?

 


 

Actually it means that while comparing two media, the one with the larger refractive index is optically denser than the other. The other medium of lower refractive index is optically rarer. The speed of light is higher in a rarer medium than a denser medium. If a ray of light enters from the rarer medium to a denser medium, it slows down and it will bend towards the normal. When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.





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