Refractive Index
Refractive Index
Refractive Index is a physical property of solids and liquids. It gives an idea about the speed of light while travelling in a different medium. Whenever the light travel from one medium (rarer) to another medium (denser), the direction and speed of light rays changes. This change in speed from one medium to another is what causes light rays to bend because as light travels through denser medium from rarer medium, the atoms of denser medium constantly absorb and reemit the particles of light, slowing down the speed light travels at.
The extent of change in the direction of light rays is calculated as refractive index. The ratio of the velocities or speed of light in different medium gives the refractive index. Upon travelling from a rarer medium to denser the light rays tend to bend towards the normal and if it travels from denser to rarer then it bends away from the normal at the point of incidence. The refractive index (n) can be calculated by using this equation-
where t = temperature in °C and D = the wavelength of the light used in nm.
Refractive index can be calculated by another method also using Snell's law.
n = sin i/sin r
From law of refraction-
where n1 and n2 are the index of refraction of the rarer and the denser medium respectively.
The angle of incidence can never be evidently greater than 90° and when it is 90° the above equation becomes-
According to wave theroy of light, the ratio of the sines of the angles of incidences and refraction is identical with the ratio of the speeds or velocities of light in the two medium. Thus-
Refractive index of any other medium is defined relative to the refractive index of a vacuum, which is assigned a value of 1. Thus, a refractive index of 1.33 for water means that light travels 1.33 times faster in a vacuum than in water. Higher refractive index lowers the speed of light.
Refractive index is defined as the ratio of two velocities. Thus, we can say that the refractive index is a dimensionless or unitless quantity.
Why do different substances have different refractive indices?
Refractive index of a material depends upon its chemical composition or nature of material, hence different substances have different refractive indices.For example, light travels in water at 3/4 of its velocity in a vacuum, so we assign a value of 4/3 or 1.333 as the index of refraction of water. But if we freeze that water into ice, a different arrangement of the molecules results and the ice has a refractive index of 1.313. Or, if we boil the water, yet another arrangement of the same molecules is produced, which retards the light very little, and the steam has a refractive index of 1.0002.
Types of Refractive Index
Refractive index depends upon the two mediums in which the light is travelling. Therefore, Refractive index is of two types-
1. Absolute Refractive Index: It has one material medium and one vacuum in which the speed of light is 3 × 108 m/s.
2. Relative Refractive Index: It is the relative change in speed or velocity of light upon travelling from one given medium to another.
Factors Affecting the Refractive Index
The two factors which affect the value of the refractive index are-
1. Temperature
Refractive index values are usually determined at standard temperature. At high temperature, the liquid becomes less dense and less viscous, causing light to travel faster in the medium. This results in a smaller value for the refractive index due to a smaller ratio. At lower temperature, the liquid becomes denser and has a higher viscosity, causing light to travel slower in the medium. This results in a larger value for the refractive index due to a larger ratio.
2. Wavelength of light
Refractive index varies with wavelength linearly because different wavelengths interfere to different extents with the atoms of the medium. Monochromatic light is used to prevent dispersion of light into different colours. The chosen wavelength should not be absorbed by the medium. The sodium D line at 598 nm is the most frequently used wavelength of light for a refractometer.
Calculation of Refractive Index from the Speed of Light
Refractive Index can easily be calculated when the speed of light is given in two mediums.
Let's consider-
Speed of Light (in 1st medium) = v1
Speed of Light (in 2nd medium) = v2
Now, the refractive index (n) of 2nd medium with respect to 1st medium is given by-
n21 = Speed of Light in 1st medium / Speed of light in 2nd medium
n21 = v1/v2
Now, the refractive index (n) of 1st medium with respect to 2nd medium is given by-
n12 = Speed of Light in 2nd medium / Speed of light in 1st medium
n12 = v2/v1
The absolute refractive index of benzene is 1.50. Calculate the speed of light in benzene.
we know that-Refractive Index (n) = Velocity of light(C) / Velocity in medium(V)
Given-
Refractive index of benzene n = 1.5
Velocity of light(C) = 3 × 108 m/s
Now putting these values in the above equation, we get-
n = 3 × 108 / V
V = 3 × 108 / n
V = 3 × 108 / 1.5
V = 2 × 108 m/s
Hence, the velocity or speed of light in benzene is 2 × 108 m/s
Q. Calculate the molar refraction of acetone at 20°C. The refractive index and density of acetone at 20°C are 1.3591 and 0.7910 g cm−3 respectively.
Answer: 16.147 cm3mol−1Applications on Refractive Index
Some Important applications of the Refractive Index are given below-
1. Refractive index is widely used for identifying a particular substance, confirming its purity, or measuring the given concentration.
2. It is used for measuring the concentration of the solute in an aqueous solution.
3. In the pharmacy industry, it is used in the determination of the drug concentration .
4. Refractive index is generally applied for the estimation of the thermophysical properties of many hydrocarbons and petroleum mixtures.
5. It is widely used to calculate the focusing power of various lenses, and the dispersive power of prisms.
