CBSE Class 10 Physics is a tricky subject like mathematics that many students fear. It is a blend of theoretical and practical concepts. Rather than developing a fear, the students should try to clarify the core concepts better. It will help them to understand the topics with ease and let go of their fear of physics from their mind. For this, a comprehensive and learning-oriented study material plays a significant role. We have high-quality study resources to help the students with efficient preparations for the final exams. Students will find our explanations very helpful to achieve success in the CBSE Class X boards. Our Class 10 Physics Concepts will also help students for NTSE, Olympiad, and other competitive exams. Today we will review one of the critical topics in physics, i.e., the Refractive Index. Now we will dive deeper to gain a good understanding of what is the Refractive Index?
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What is the Refractive Index?
Refractive Index measures the bending of a ray of light while it enters from one medium into another medium. In simpler words, The Refractive Index measures the change in speed of light when it passes into a medium from the air.
The Refractive Index is also known as the Index of Refraction.
The extent of change in speed of light in a medium depends upon the refractive index of that particular medium.
Fact Review
Refractive Index is the property that enables the construction of lenses, such as the lens used in cinema projectors, which focus the light to form real images.
How to Calculate Refractive Index?
We have the formula to calculate the refractive index of a definite medium.
Before jumping on to it, we must go through a short explanation regarding how the formula for refractive index is derived. Let us begin! In case you are looking for previous year’s papers, access Class 10 Previous Year Question Papers here!
Let us imagine a simple graph where a horizontal is a bifurcation mark for two mediums, medium I and medium II. You can further suppose medium I as vacuum and medium II as air/water. Also, there is a vertical line perpendicular to the surface of the mediums. It is known as the Normal. Now, assume a ray of light passing from medium I to medium II. While entering into medium II, the ray of light will bend. The Refractive Index measures this change.
Putting light on the derivation,
Let i be the angle of incidence of a ray of light in a vacuum (medium I). It is the angle between the incoming light ray and the Normal line perpendicular to the surface. Then, r is the angle of refraction. It is the angle between the ray in medium II (air) and the normal.
Here, n symbolises the Refractive index. n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction. i.e.,
n = sin i / sin r
Also, the Refractive index equals the velocity of light of a given wavelength in a space (c) divided by its velocity (v) in a substance. So, we can represent n as:
Refractive Index = Velocity of light in vacuum/ Velocity in a medium
Or,
This implies that,
n = c/v
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Some Common Refractive Indices
Different mediums have different Refractive Indices. The higher the refractive index of a medium, the slower is the speed of light. It causes a corresponding increased change in the direction of the light travelling within the medium.
If the refractive index of a particular medium is 1.2, it means the speed of light in that medium is 1.2 times slower than the speed of light in a vacuum. The speed of light in glass is independent of the colour of light.
Here are some common refractive indices for yellow light whose wavelength is equal to 589 nm, which are tabulated below:
Note: 1 nanometer = 10-9 metre
Medium | Refractive Index |
Air | 1.0029 |
Water | 1.333 |
Ice | 1.31 |
Kerosene | 1.44 |
Ethanol | 1.35 |
Crown glass | 1.517 |
Dense flint glass | 1.655 |
Sodium chloride | 1.54 |
Diamond | 2.417 |
Fact Review
The medium having the highest refractive index of 2.417 is Diamond. It implies that the light-bending ability of diamond is approximately 2.42 times more than air.
Types of Refractive Index
Mainly there are two types of Refractive Index, i.e.,
- Absolute Refractive Index
- Relative Refractive Index
Absolute Refractive Index
The Absolute Refractive Index is the refractive index in a vacuum. It is the ratio of the velocity of light in a vacuum to the velocity of light in another medium.
(This is similar to what we discussed above in the calculation of the Refractive Index portion.)
Absolute Refractive Index (n) = c/v
where c is the velocity of light in the vacuum and v in the medium.
The Absolute Refractive Index has one material medium, and one is the vacuum. Notably, the speed of light in a vacuum is 3.00 × 108 m/s.
Relative Refractive Index
The Relative Refractive Index is the ratio of the velocity of light in one medium to the velocity of light in another medium.
In other words, the Relative Refractive Index is the relative change in speed or velocity of light while travelling from one specific medium to another specific medium.
Notably, the relative refractive index between two media is the ratio of their absolute refractive indices.
While considering the Relative Refractive Index, we have two different mediums, i.e. Medium A and Medium B.
Then, the calculation of the Refractive Index will be:
- Refractive Index of Medium B = Refractive index of Medium B concerning that of Medium A.
Formula:
nBA = Velocity of Light in Medium A / Velocity of Light in Medium B
- Refractive Index of Medium A = Refractive index of Medium A concerning that of Medium B.
Formula:
nAB = Velocity of Light in Medium B / Velocity of Light in Medium A
Also, in the case of calculating the Refractive Index, one medium is always denser than another medium, or vice versa.
- i) When light travels from a rarer to a denser medium, the angle of incidence is greater than the angle of refraction. So, the light ray bends towards the normal.
Here, ∠i > ∠r
And,
Please note: The Relative Refractive Index of the denser medium concerning the rarer medium will be greater than 1.
- ii) When light travels from a denser to a rarer medium, the angle of incidence is smaller than the angle of refraction. In this case, the light ray bends away from the normal.
Here, ∠i < ∠r
Fact Review
The higher the refractive index of a medium, the higher is the optical density of that medium.
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Colours And Refractive Index
Let us explore the concept of the Refractive Index concerning colours.
In a vacuum, all the colours have similar velocities. But when they pass from a vacuum into any other medium, their velocity changes. Hence, colours have different refractive indices.
Now, the refractive index varies with different wavelengths, as in the dispersion of white light from a prism, different colours of light travel at different speeds. All the colours get bent by different amounts and spread out distinctly rather than mixed up.
Violet colour has the highest refractive index, so it travels the slowest and is at the bottom among the seven colours. On the other hand, the Red colour has the lowest refractive index, so red travels the fastest and is at the top.
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Conclusion
The Refractive index represents to what extent a ray of light changes direction when it travels from one medium to another. When light travels between different transparent media, refraction takes place. It is a result of their varying optical properties. As light travels across a medium, it changes its speed as per the nature of a particular medium. Refractive Index is a material property that shows how a definite material affects the speed of light that travels through it. We hope this blog proves to be useful for you. You can also get NCERT Solutions for Class 10 here.
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