Class 10 Science Solutions English Medium

Chapter 10. Light-Reflection and Refraction

Refraction of light: 

When a ray of light enters from one transparent medium to another transparent medium it changes its direction or it bends. This bending of light path is called refraction of light. 

The refraction of light only takes place through transparent materials like glass, air and water etc. 

There are two situation of this bending.

(i) When a ray of light enters from rarer medium to denser medium it bends towards the normal. 

(ii) When a ray of light enters from denser medium to rarer medium it bends away from normal. 

Cause of the refraction of light: The refraction of light occurs due to changing in speed of light it enter on transparent medium to another.  

The Law of refraction: 

The following are the laws of refraction of light:

(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

Snell's Law: The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

Denser Medium: The medium which has greater refractive index is called dender medium. The particles of this medium is denser.

Rarer Medium: The medium which has smaller refractive index is called rarer medium. 

Refractive Index: The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This constant value is called refractive index of the second medium with respect to the first medium. 

• The refractive index can be linked to an important physical quantity, the relative speed of propagation of light in different media. It turns out that light propagates with different speeds in different media. Light travels the fastest in vacuum with the highest speed of 3×108 m s–1.
• In air, the speed of light is only marginally less, compared to that in vacuum.
• It reduces considerably in glass or water. The value of the refractive index for a given pair of media depends upon the speed of light in the two media.
• Refractive index with higher value of material has less speed of light through it.
• Refractive index with lower value of material has high speed of light through it.

Example: The refractive index of diamond is 2.42, it means the ratio of speed of light in air and speed of light in diamond is 2.42. Diamond has higher refractive index and when the light will pass through it, the speed of light will be less. The material media of higher refractive index reduce the speed of light than normal and the material media of lower refractive index increase the speed of light when pass through it. 

Absolute refractive index of some material media:

 An optically denser medium may not possess greater mass density. For example, kerosene having higher refractive index, is optically denser than water, although its mass density is less than water.

Optically Denser Medium: In comparing two media, the one with the larger refractive index is optically denser medium than the other.

Optically Rarer Medium: In comparing two media, the one with the lower refractive index is optically rarer medium than the other.

Refractive Index of Medium₂ with respect to Medium₁:

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 is usually represented by the symbol n₂₁.

Example: The speed of light in air is 3 x 10⁸ ms^-1 and the speed of light in water is 2.25 x 10⁸ ms^-1. Find the refractive index of water with respect to air.

Solution: Speed of light in air = 3 x 10⁸ ms^-1. 

Speed of light in water - 2.25 x 10⁸ ms^-1. 

Refractive Index of Water is 1.33 

Refractive Index of Medium₁ with respect to Medium₂:

The refractive index of medium1 with respect to medium 2 is represented as n₁₂. It is given by

Example: Speed of light in water is 2.25 x 10⁸ ms^-1 and the speed of light in Kerosene is 2.08×10⁸ ms^-1. Find the Refractive index of water with respect to Kerosene. 

Solution: Speed of light in water (V₁) = 2.25 x 10⁸ ms^-1. 

Speed of light in Kerosene (V₂) = 2.08 x 10⁸ ms^-1. 

Refractive Index of water = Speed of light in Kerosene (V₂)/Speed of light in water (V₁)

= 2.08 x 10⁸ ms^-1/2.25 x 10⁸ ms^-1

= 2.08/2.25 

= 0.924 

Refractive Index of water with respect to Kerosene = 0.924 

Questions for Practice: 

Q1. What is refractive index? 

Q2. Write the law of refraction of light.

Q3. What are the two situation of bending the path of light during refraction of light.

Q4. The refractive index of diamond is 2.42. What is the meaning of this statement?

Q5. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 10⁸ m s^–1.

Q6. Define Optically denser medium and Optically rarer medium. 

Q7. Write the Snell's law. 

Q8. The Refractive Index Of Kerosene Is 1.44 . Find The Speed Of Light In Kerosene.

Q9. You are given kerosene, turpentine and water. In which of these does the light travel fastest?

Q10. Give the examples of the medium having highest optical density.
Also give the example of the medium with lowest optical density.