1. The change in the _______ of light rays while going from one medium to another is called refraction.
Answer :direction
The change in the direction of light rays while going obliquely from one transparent medium to another of different optical density is called refraction of light. It occurs because the speed of light changes between different media.
2. Mirage is an effect of _______.
Answer :atmospheric refraction and total internal reflection
On hot summer days, ground air layers are much hotter and less dense than upper cooler layers. Light from distant objects travels from denser to rarer air, bending away from the normal until the angle of incidence exceeds the critical angle, undergoing total internal reflection and producing the inverted illusion of a mirage.
3. The absolute refractive index of a medium is defined as the ratio of _______.
Answer :velocity of light in vacuum to the velocity of light in that medium
n = c / v, where c is the speed of light in vacuum (3 × 10⁸ m/s) and v is the speed of light in the given medium.
Q2)State whether the following statements are True or False. If false, write the correct statement:
1. When a ray of light passes from a rarer medium to a denser medium, it bends away from the normal.
Answer :False.
Correct Statement: When a ray of light passes obliquely from an optically rarer medium to an optically denser medium, its speed decreases and it bends towards the normal.
2. Red light bends the least while violet light bends the most during dispersion through a prism.
Answer :True.
Explanation: Red light has the longest wavelength (λ ≈ 700 nm) and travels fastest in glass, having the lowest refractive index; hence it deviates the least. Violet light has the shortest wavelength (λ ≈ 400 nm) and deviates the most.
3. During total internal reflection, the angle of incidence must be greater than the critical angle.
Answer :True.
Explanation: Total internal reflection occurs only when light travels from an optically denser to a rarer medium and the angle of incidence strictly exceeds the critical angle (i > ic).
Q3)Write scientific reasons:
1. Stars twinkle at night, but planets do not twinkle.
Answer :
Stars are situated billions of kilometers away and act as effective point sources of light.
As starlight traverses the Earth’s atmosphere, it passes through continuously fluctuating layers of air with differing temperatures and refractive indices.
This causes continuous atmospheric refraction. The apparent position of the star shifts slightly, and the intensity of light entering our eyes varies constantly, making stars appear to twinkle.
In contrast, planets are much closer to Earth and act as extended sources (collections of numerous point sources). The fluctuating intensities from different points cancel each other out, so planets shine with steady brightness.
2. We see the Sun about two minutes before the actual sunrise and for about two minutes after actual sunset.
Answer :
When the Sun is just below the horizon, rays of light travel from outer space (vacuum) into progressively denser layers of the Earth’s atmosphere.
The light bends continuously towards the normal due to gradual atmospheric refraction.
To an observer on Earth, these refracted rays appear to come from above the horizon.
Hence, the Sun becomes visible about 2 minutes before actual sunrise and remains visible for about 2 minutes after actual sunset, lengthening daylight by approximately 4 minutes.
3. White light disperses into seven colours when passed through a glass prism.
Answer :
White light is composed of seven constituent spectral colors: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
In vacuum, all colors travel at the identical speed (3 × 10⁸ m/s). However, in a transparent medium like glass, light of different wavelengths travels with different velocities.
The refractive index of glass is highest for violet (shortest wavelength) and lowest for red (longest wavelength).
Therefore, each color bends at a different angle, separating into a colorful spectrum.
Q4)Solve the following numerical problems:
1. The absolute refractive index of water is 1.36. What is the velocity of light in water? (Velocity of light in vacuum = 3 × 10⁸ m/s).
Answer :
Given: Speed of light in vacuum c = 3 × 10⁸ m/s, Refractive index of water n = 1.36
Formula:n = c / v ⇒ v = c / n
v = (3 × 10⁸) / 1.36 ≈ 2.21 × 10⁸ m/s
The velocity of light in water is 2.21 × 10⁸ m/s.
2. The absolute refractive index of diamond is 2.42. What is the speed of light in diamond?
Answer :
Given:c = 3 × 10⁸ m/s, n = 2.42
Formula:v = c / n
v = (3 × 10⁸) / 2.42 ≈ 1.24 × 10⁸ m/s
The speed of light in diamond is 1.24 × 10⁸ m/s.
3. 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 :
Given: Refractive index of glass ng = 3/2 = 1.5, Refractive index of water nw = 4/3 ≈ 1.33
Formula: Refractive index of glass w.r.t water: wng = ng / nw
wng = (3/2) / (4/3) = (3/2) × (3/4) = 9/8 = 1.125
The refractive index of glass with respect to water is 9/8 (or 1.125).
Q5)Explain the formation of a rainbow with the help of a neat labelled diagram:
Answer :
A rainbow is a beautiful natural optical spectrum caused by a combined sequence of refraction, dispersion, and total internal reflection of sunlight by microscopic water droplets suspended in the atmosphere:
Refraction and Dispersion: When a parallel beam of white sunlight enters a spherical raindrop, it refracts at the front surface and disperses into its constituent seven spectral colors (VIBGYOR).
Internal Reflection: The dispersed rays travel to the back inner wall of the droplet. Since their angle of incidence exceeds the critical angle for water (approx. 48.6°), they undergo total internal reflection inside the drop.
Refraction out of Droplet: Upon reaching the front surface again, the rays refract out into the air, fanning out towards an observer's eyes with red light at an angle of ~42° and violet light at ~40°.
Fig 6.1: Internal Reflection and Dispersion inside a Raindrop (Rainbow Formation)
Q6)Prove that the angle of incidence is equal to the angle of emergence for a light ray passing through a rectangular glass slab:
Answer :
Let a light ray undergo refraction through a rectangular glass slab of parallel faces:
At the top air-glass boundary (refraction from air into glass):
ang = sin i / sin r ...(Equation 1)
At the bottom glass-air boundary (refraction from glass into air):
gna = sin r₁ / sin e ...(Equation 2)
By the principle of reversibility of light:
gna = 1 / ang ⇒ ang = sin e / sin r₁ ...(Equation 3)
Because the two opposite faces of the slab are parallel, the normals are parallel, making the interior alternate angles equal: r = r₁.
Equating Equation 1 and Equation 3:
sin i / sin r = sin e / sin r₁
Since r = r₁, their sines cancel out: sin i = sin e ⇒ i = e.
Angle of Incidence (i) = Angle of Emergence (e)
The emergent ray is parallel to the incident ray, displaced laterally by a distance called lateral displacement.
Fig 6.2: Refraction of Light through a Rectangular Glass Slab (i = e)
Class 10 Maharashtra State Board Science 1 Chapter 6 covers the laws of refraction of light, Snell's Law (sin i / sin r = constant), absolute and relative refractive indices, refraction through a glass slab, atmospheric refraction effects (twinkling of stars, mirage, advanced sunrise and delayed sunset), dispersion of white light into spectrum colors (VIBGYOR), critical angle, total internal reflection, and rainbow formation.
Key Formulas Summary
Snell's Law:n = sin i / sin r
Absolute Refractive Index:n = c / v where c = 3 × 10⁸ m/s
Relative Refractive Index:¹n₂ = v₁ / v₂ = n₂ / n₁
Total Internal Reflection Condition: Light travels from denser to rarer medium and i > ic
Frequently Asked Questions (FAQs)
Q1. Why does light bend when entering a different medium?
Because the speed of light changes between media of differing optical densities. The wave fronts change velocity, causing the direction of propagation to bend.
Q2. What is the critical angle?
The angle of incidence in an optically denser medium for which the angle of refraction in the rarer medium is exactly 90° is called the critical angle.
Q3. Why do stars appear higher in the sky than they actually are?
Due to continuous atmospheric refraction towards the normal as starlight enters progressively denser atmospheric layers, the apparent position of the star is slightly above its true position.
Q4. What is lateral displacement?
The perpendicular distance between the original incident ray path and the emergent ray path when light passes through a parallel-sided refracting medium like a glass slab.
Related Study Topics
Refraction of Light Class 10Snell's Law FormulaGlass Slab Ray DiagramTwinkling of Stars ReasonRainbow Formation DiagramTotal Internal ReflectionDispersion of LightSSC Board Science 1 Solutions