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Chapter 2 – Refraction Of Light

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Have you ever thought, why does the bottom of the tank having water appear to be a little bit raised? Why does a pencil partially immersed in water appear displaced from its original position and different in shape also? Why do stars twinkle in the sky?

The answer to all the above questions is refraction. This happens when light travels from one medium to another medium, i.e., the medium should be different.

Introduction

When light obliquely (in a slanting manner) travels from one medium to another, then the direction of light changes in the second medium. This phenomenon is called the refraction of light.

For example, when a coin is kept at the bottom of water-filled transparent glass, and someone tries to see it from outside, the coin appears a little bit raised. This happens because light travels in two different mediums, i.e., air and water. Firstly, light travels from air to water, then it travels from water to air.

Today, there are numerous optical instruments we use, like a magnifying glass, prism, lenses, and so on, due to light’s refraction phenomenon.

There are two laws of refraction of light.

i. The incident ray (XO), the refracted ray (OY), and normal (NN’) at the interface of two mediums all lie in the same plane.

ii. The ratio of the sine of the angle of incidence (∠i) to the sine of the refraction angle (∠r) is constant for the given pair of media. This law is also known as Snell’s law of refraction.

\frac{Sin i}{Sin r} = constant

Here, “constant” is known as the refractive index of the second medium.

The Refractive index is the ratio of the speed of light in the first medium, and the speed of light in the second medium is known as the refractive index. It is a unitless quantity.

Absolute refractive index– It is the refractive index of a medium with respect to a vacuum represented by n2.

\frac{Speed of light in air}{Speed of light in medium} = n_2

Here, the speed of light in the air can be denoted as “c,” and the speed of light in the medium can be denoted as “v.” 

Again, the refractive index of medium 1 with respect to medium 2 is represented as;

\frac{Speed of light in medium 2 (V2)}{Speed of light in medium 1 (V1)} = n_{12}

and vice-versa.

Numerical: Light enters from air to glass, having a refractive index of 1.50. What is the speed of light in the glass? The speed of light in vacuum = 3 × 108 ms-1

Solution: According to the question,

Speed of light in vacuum = 3 × 108 ms-1

Refractive index of glass = 1.50

We know,

Refractive index of a medium  = Speed of light in vacuum/Speed of light in the medium

then,

Refractive index of glass = Speed of light in vacuum/Speed of light in the glass

Speed of light in the glass = Speed of light in vacuum/ Refractive index of glass

Speed of light in glass = (3 × 108)/1.50

Speed of light in glass = 2 × 108 ms-1

Factors on which the refractive index of a medium depend is given below:

i. Nature of the surrounding medium

ii. Nature of the medium

iii. Temperature

iv. Wavelength of light

The refractive index of the given pair of mediums does not depend upon the angle of incidence. The medium having a larger refractive index is an optically denser medium and vice-versa.

Refractive index of some medium

Air – 1.0003 Water – 1.33 Diamond – 2.42 Rock salt – 1.54

Sapphire – 1.77 Ice – 1.31 Kerosene – 1.44 Ruby – 1.71

The refraction causes a change in speed and change in direction, which is explained below.

  • When light rays travel from rarer to a denser medium, its speed slows down and bends towards the normal.
  • When light rays travel from denser to a rarer medium, its speed increases and bends away from the normal.

i. Due to the refraction of light, the following phenomenon occurs:

ii. Twinkling of the star at night in the sky

iii. Formation of rainbow

iv. White color light split into seven colors when refracts through prism

v. The phenomenon of refraction is widely used in optics and technology like a lens, magnifying glass, and so on.

Case 1– When light travels from air to glass, i.e., rarer to a denser medium

As light ray travels from rarer to a denser medium, its speed decreases, and ray bends toward normal.

Case 2– When a light ray travels from glass to air, i.e., denser to rarer medium

When a light ray travels from denser to rarer medium, its speed increases and the ray bends away from the normal.

  1. What is meant by the refraction of light?

Ans. It is a phenomenon of light that occurs when light travels from two different mediums, like air to water, air to sapphire, etc.

  1. Why does light refraction occur?

Ans: Light refraction occurs due to change in medium, i.e., denser to rarer medium or rarer to denser medium.

  1. What are the three laws of refraction?

Ans. The two laws of refraction of light are:

i. The incident ray (XO), the refracted ray (OY), and normal (NN’) at the interface of two mediums all lie in the same plane.

Ii. The ratio of the sine of the angle of incidence (∠i) to the sine of the angle of refraction (∠r) is constant for the given pair of media. This law is also known as Snell’s law of refraction.

\frac{Sin i}{Sin r} = constant

Here, “constant” is known as the refractive index of the second medium.

  1. What’s another word for refraction?

Ans. Diversion or Deflection

  1. What is the difference between refraction and diffraction?

Ans. Refraction of Light – Refraction of light takes place when light travels from two different mediums. This happens due to the difference in the refractive index of the pair of media.

Diffraction of light – When a light ray crosses through the edge of an object, it bends slightly. This is called the diffraction of light. 

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