Std 10th Science Part 1 – Chapter 6 : Refraction of Light Exercise Questions And Answers Maharashtra Board

Q1. Fill in the blanks and Explain the completed sentences.

a. Refractive index depends on the velocity of light.

Explanation: The refractive index (n) of a medium is defined by the ratio of the velocity of light in the first medium (v1​) to the velocity of light in the second medium (v2​). Since the velocity of light changes when it travels from one medium to another, the refractive index tells you how much the light slows down and changes direction.

b. The change in the direction of propagation of light rays while going from one medium to another is called refraction.

Explanation: Refraction happens because light travels at different speeds in different transparent media. When light moves from one medium (like air) to another (like glass or water), it changes its path or direction.

Q2. Prove the following statements.

a. If the angle of incidence and angle of emergence of a light ray falling on a glass slab are i and e respectively, prove that, i=e

Answer – 

Proof:

1. Consider a light ray passing through a rectangular glass slab.
2. At the first surface (air to glass), the angle of incidence is
   i, and the angle of refraction is r.
3. At the second surface (glass to air), the angle of incidence is
   i1​, and the angle of emergence is e.
4. Since the two parallel sides of the glass slab (PQ and SR) are parallel, the normals drawn at the points of incidence and emergence are also parallel. Therefore, the refracted ray at the first surface is the incident ray at the second surface, and the angle of refraction at the first surface is equal to the angle of incidence at the second surface:
   r=i_1​.
5. The first law of refraction (Snell’s Law) for both refractions gives an equal change in direction but in opposite directions.

As a result, the emergent ray is parallel to the incident ray. In a parallel-sided glass slab, the angle of incidence is equal to the angle of emergence: i=e.

b. A rainbow is the combined effect of the refraction, dispersion, and total internal reflection of light. 

• Proof/Explanation:
  1. A rainbow is a natural phenomenon seen after rainfall, produced by tiny water droplets acting as small prisms.
  2. Refraction and Dispersion: When sunlight enters a water droplet, it first undergoes refraction (bending) and dispersion (splitting into its component colors: VIBGYOR).
  3. Total Internal Reflection: These dispersed colors travel inside the droplet and hit the inner back surface, where they undergo total internal reflection.

Refraction and Emergence: Finally, the reflected and dispersed light is refracted again when it comes out of the droplet, leading to the formation of the rainbow spectrum we see.

Q3. Mark the correct answer in the following questions.

A. What is the reason for the twinkling of stars?

i . Explosions occurring in stars from time to time 

ii. Absorption of light in the earth’s atmosphere 

iii. Motion of stars

iv. Changing refractive index of the atmospheric gases

Answer – 

• iv. Changing refractive index of the atmospheric gases
  • Reason: The atmosphere’s density, temperature, and refractive index constantly change due to air motion. This continuous change makes the star’s light change direction and brightness, causing it to twinkle.

B. We can see the Sun even when it is little below the horizon because of 

i. Reflection of light 

ii. Refraction of light 

iii. Dispersion of light 

iv. Absorption of light

Answer – 

• ii. Refraction of light

Reason: When the Sun is slightly below the horizon, its light rays reach us by bending along a curved path due to refraction through the Earth’s atmosphere. This makes the Sun appear higher than its actual position.

C. If the refractive index of glass with respect to air is 3/2, what is the refractive index of air with respect to glass?

Answer – 

d. 2 / 3​Reason: The refractive index of medium 1 with respect to medium 2 (n_12​) is the inverse of the refractive index of medium 2 with respect to medium 1 (n₂₁​).
n_{glass with respect to air} ​= 3 / 2​
n_{air with respect to glass​}= _​1​ / n_{glass with respect to air} = ​1 / 3/2 ​ = 2​ / 3

Q4. Solve the following examples.

a. If the speed of light in a medium is 1.5 x 10⁸ m/s, what is the absolute refractive index of the medium?

Answer – 

• Formula to use: Absolute refractive index (n) = Velocity of light in medium (V₂​)Velocity of light in vacuum (V_1​)​
• Given:
  • Velocity of light in vacuum (V₁​) ≈ 3×108 m/s
  • Velocity of light in medium (V₂​) = 1.5×108 m/s
• Calculation:
  n = 3×10⁸ m/s​ /  1.5×10⁸ m/s = 2
• Answer: The absolute refractive index of the medium is 2.

b. If the absolute refractive indices of glass and water are 3/2 and 4/3 respectively, what is the refractive index of glass with respect to water?

Answer – 

• Formula to use: Refractive index of glass (g) with respect to water (w) =  Absolute refractive index of glass (n_g​)​ / Absolute refractive index of water (n_w​)
• Given:
  • Absolute refractive index of glass (n_g​) = 3 / 2​
  • Absolute refractive index of water (n_w​) = 4 / 3​
• Calculation:

Answer: The refractive index of glass with respect to water is 9/8.