Refractive Index Questions

What is the Refractive Index?

The ratio of the speed of light in a vacuum to its speed in a particular medium is known as the refractive index. Refractive index is also referred to as refraction index or index of refraction. The phenomenon of refractive index is being observed almost daily since it is very common in our daily lives.

When a light ray travels from one medium to another medium, then due to the variation in the speed of light, it changes its direction because the speed of light in a medium depends on the properties of the medium.

In the case of electromagnetic waves, the speed totally hangs on the optical density of the medium in which it propagates. When atoms in a material restore the absorbed electromagnetic energy, then it is known as Optical density. If the material is more optically dense, then the speed of light will be slower in it. The refractive index is also an indicator of the optical density of a medium.

The phenomenon of refraction depends upon the following characteristics: –

• Angle of Refraction
• Speed of the light in the medium

Formula for Refractive Index

As we know, the ratio of the speed of light in a vacuum to the speed of the light in a particular medium is termed as refractive index or index of refraction; the value of the refractive index differs from one medium to another.

Therefore, the refractive index can be calculated by dividing the velocity of light in a vacuum by the velocity of light in a medium. The formula of the refractive index is given as:

n = c/v

Where,

• n is the refractive index is denoted by n
• Velocity of light in a vacuum (3 × 10⁸ m/s) is c
• Velocity of light in a substance is v

The refractive index of vacuum is always 1. The above equation can be used to calculate the refractive index of other materials. The higher the refractive index, the slower the speed of light and the higher the optical density. The table below shows the refractive index of different material mediums.

Important Questions on Refractive Index

1) Define Refractive Index.

The ratio of the speed of light in a vacuum to its speed in a particular medium is known as the refractive index. Refractive index is also termed as refraction index or index of refraction.

2) State and explain Snell’s Law.

For a given pair of media and for a given wavelength, when the sine of the angle of incidence is divided by the sine of the angle of refraction, that is, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is known as Snell’s law.

(n = sin i/sin r) where i is the angle of incidence and r is the angle of refraction.

Conditions for no refraction:

1. If the incident light is perpendicular to the surface of the two media,

2. If both the mediums have the same refractive index.

3) What is the refractive index gradient?

With respect to distance in the particular material, the rate of change of the refractive index is known as the refractive index gradient. Here, the slope of the refractive index profile at any point is the distance. Refractive index gradient is articulated in terms of reciprocal of a unit of distance, it is a vector point function.

4) What will be the refractive index of the medium in which the speed of light is 2.5 ×10⁸ m/s?

Using the formula: n = c/v

the refractive index of the medium can be calculated

Substituting the values in the formula, we get

n = 3 × 10⁸ m/s/2.5 × 108 m/s = 1.2

Therefore, the refractive index of the medium is 1.2

5) Calculate the refractive index of the medium if the speed of light in that medium is given as 2.76 × 10⁸ m/s.

Using the formula: n = c/v

the refractive index of the medium can be calculated

Speed of light in a vacuum is c

Speed of light in the medium is v

Substituting the values in the formula, we get

n = (3 × 10⁸)/ (2.76 × 10⁸) = 1.086

6) Which among the following is the cause of the twinkling of stars?

(a) Bursting of light from the stars periodically

(b) Variation of refractive index in the atmosphere

(c) Incomplete absorption of light in the atmosphere

(d) Interference of sunlight with the starlight

Correction Option: (b)

Explanation: Our atmosphere has different layers; therefore, the twinkling of stars is caused by the refraction of light through several layers of different refractive indices present in the atmosphere; due to this reason, stars appear to be twinkling in the sky.

7) What are the real-life Examples of Refraction of Light?

Here are some common daily life examples of refraction of light in more detail –

• One of the most perfect examples of refraction is the formation of a rainbow when the rays of the Sun split into its constituent colours after passing through a raindrop, which eventually results in the formation of a rainbow.
• The phenomenon of refraction of light causes two optical illusions termed as mirage and looming.
• The splitting of white light into seven colour components through a glass prism is an example of the refraction of light.
• The twinkling of stars is caused by the refraction of light through several layers of different refractive indices present in the atmosphere.

8) What are the different types of refraction?

Basically, there are 2 types of refraction –

• Refraction from denser to the rarer medium: The relative refractive index, in this case, is less than 1, and the angle of refraction is greater than the angle of incidence; that is, the refracted rays tend to shift away from the normal. For example, refraction from glass to air.
• Refraction from rarer to the denser medium: The relative refractive index, in this case, is greater than 1, and the angle of refraction is less than the angle of incidence. Therefore, the refracted rays lean towards the normal. Example: Refraction of light in water from the air.

9) What are the laws of refraction of light?

The two important laws of refraction are as follows –

• The incident ray, refracted ray and the normal should lie on the same plane.
• For a given pair of media and for a given wavelength, when the sine of the angle of incidence is divided by the sine of the angle of refraction are constant, which is also known as Snell’s law.

10) If the refractive index of water is 4/3 and that of glass is 3/2. What will be the refractive index of glass w.r.t. water?

µ_w = 4/3, and µ_g = 3/2

Refractive index of glass w.r.t. water ( ^wµ_g ) = ^aµ_{g /} ^aµ_w

= 3/2 / 4/3

= 3/2 X 3/4

= 9/8

Watch the video and solve the previous year questions in the chapter Light Reflection and Refraction Class 10

Practice Questions

1. What are the different applications of the refractive index phenomenon?
2. What is the absolute refractive index?
3. Explain the phenomenon of mirage and looming.
4. Define Snell’s law.
5. What are the laws of refraction?

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