Grade 10 Q&A: Chapter 6: Refraction of Light

Concept Questions

Q1: What is 'refraction of light'?

Answer: Refraction of light is the phenomenon of the bending of light rays as they pass from one transparent medium to another, caused by a change in the speed of light.

Q2: What is the main cause of refraction of light?

Answer: The main cause of refraction of light is the change in the speed of light as it travels from one medium to another.

Q3: State Snell's Law of refraction.

Answer: Snell's Law states that for a given pair of media and a given color of light, the ratio of the sine of the angle of incidence (sin i) to the sine of the angle of refraction (sin r) is constant. Mathematically, sin i / sin r = constant (n₂), where n₂ is the refractive index of the second medium with respect to the first.

Q4: What is the 'refractive index' of a medium?

Answer: The refractive index (n) of a medium is a measure of how much the speed of light is reduced when it passes through that medium. It is defined as the ratio of the speed of light in vacuum to the speed of light in the medium (n = c/v).

Q5: What is 'absolute refractive index'?

Answer: Absolute refractive index is the refractive index of a medium with respect to vacuum or air. It is always greater than 1.

Q6: When does a light ray bend towards the normal during refraction?

Answer: A light ray bends towards the normal when it passes from an optically rarer medium (e.g., air) to an optically denser medium (e.g., glass or water).

Q7: When does a light ray bend away from the normal during refraction?

Answer: A light ray bends away from the normal when it passes from an optically denser medium (e.g., glass or water) to an optically rarer medium (e.g., air).

Q8: What is 'dispersion of light'?

Answer: Dispersion of light is the phenomenon of splitting of white light into its constituent colors (spectrum) when it passes through a transparent medium like a prism, due to different colors having different speeds and thus different refractive indices in that medium.

Q9: Name the colors in the spectrum of white light in order.

Answer: The colors in the spectrum of white light in order are Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).

Q10: What is 'Total Internal Reflection (TIR)'?

Answer: Total Internal Reflection (TIR) is the phenomenon where a ray of light traveling from an optically denser medium to an optically rarer medium, at an angle of incidence greater than the critical angle, is completely reflected back into the denser medium.

Q11: What is the 'critical angle'?

Answer: The critical angle is the specific angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90 degrees. If the angle of incidence exceeds the critical angle, total internal reflection occurs.

Q12: Name two natural phenomena caused by atmospheric refraction.

Answer: Two natural phenomena caused by atmospheric refraction are the twinkling of stars and the apparent flattening of the Sun at sunrise/sunset.

Q13: What is a 'rainbow'?

Answer: A rainbow is a natural spectrum appearing in the sky after a rain shower, caused by the dispersion, refraction, and total internal reflection of sunlight by tiny water droplets suspended in the atmosphere.

Q14: What is an 'optical fiber'?

Answer: An optical fiber is a thin, transparent fiber, usually made of glass or plastic, that transmits light over long distances using the principle of total internal reflection.

Q15: What happens to a light ray incident normally on a surface separating two media?

Answer: A light ray incident normally (perpendicularly) on a surface separating two media does not deviate from its path; it passes straight through without bending.

Application-Based Questions

Q16: Explain why a stick partially immersed in water appears bent at the water surface.

Answer: A stick partially immersed in water appears bent due to refraction of light. Light rays coming from the submerged part of the stick travel from water (denser medium) to air (rarer medium). As they exit the water, they bend away from the normal. When these refracted rays reach our eyes, our brain traces them back in straight lines, making the submerged part of the stick appear to be at a shallower and different position than its actual location, thus causing the illusion of bending.

Q17: Why does a swimming pool appear shallower than it actually is?

Answer: A swimming pool appears shallower than it actually is due to refraction of light. Light rays coming from the bottom of the pool travel from water (denser medium) to air (rarer medium). As they cross the water-air interface, they bend away from the normal. When these refracted rays reach our eyes, our brain perceives them as originating from a point higher than the actual bottom, making the pool appear shallower (apparent depth is less than real depth).

Q18: How are optical fibers used in communication?

Answer: Optical fibers are widely used in communication for transmitting data (voice, video, internet) over long distances with high speed and minimal loss. Light signals, representing data, are sent through the core of the optical fiber. Due to the principle of total internal reflection, the light rays repeatedly reflect off the inner walls of the fiber, staying confined within the core and traveling along its length, thus carrying information efficiently.

Q19: Explain why stars twinkle but planets do not.

Answer: Stars twinkle because they are very distant point sources of light. As light from a star passes through the Earth's turbulent atmosphere (which has varying refractive indices due to changing air density and temperature), it undergoes continuous and random refraction. This causes the apparent position and brightness of the star to fluctuate rapidly, leading to twinkling. Planets, being much closer, appear as extended sources. Light from different points of a planet undergoes similar but slightly different refractions, and the average effect cancels out, so planets do not appear to twinkle significantly.

Q20: Why does the Sun appear flattened at sunrise and sunset?

Answer: The Sun appears flattened at sunrise and sunset due to atmospheric refraction. When the Sun is near the horizon, its rays pass through a much thicker layer of the Earth's atmosphere compared to when it is overhead. The atmosphere acts like a lens, refracting the light. The lower edge of the Sun's disc experiences more refraction than the upper edge because its light travels through a denser atmospheric path. This differential refraction causes the apparent vertical diameter of the Sun to be compressed, making it appear flattened.

Higher-Order Thinking Questions

Q21: Describe the two conditions necessary for Total Internal Reflection (TIR) to occur.

Answer: Two conditions are necessary for Total Internal Reflection (TIR) to occur: 1. The light ray must be traveling from an optically denser medium to an optically rarer medium (e.g., from water to air, or glass to air). 2. The angle of incidence in the denser medium must be greater than the critical angle for that pair of media.

Q22: Why does dispersion occur when white light passes through a prism, but not through a rectangular glass slab?

Answer: Dispersion occurs when white light passes through a prism because: 1. Different colors (wavelengths) of light travel at slightly different speeds in the prism material, meaning they have different refractive indices. 2. The two refracting surfaces of a prism are inclined to each other, so the deviation produced by the first surface is further enhanced by the second surface, causing the colors to separate. In a rectangular glass slab, although dispersion occurs at the first surface, the second surface (parallel to the first) reverses the effect, and the emergent rays are parallel to the incident rays, so the colors recombine, and no net dispersion is observed.

Q23: How does the formation of a rainbow involve the phenomena of dispersion, refraction, and total internal reflection?

Answer: Rainbow formation is a beautiful display involving all three phenomena: 1. Refraction & Dispersion: Sunlight enters a spherical rain droplet. As it enters, it undergoes refraction and simultaneously disperses into its constituent colors because each color bends at a slightly different angle. 2. Total Internal Reflection (TIR): The dispersed light rays travel to the back inner surface of the droplet. If the angle of incidence at this surface is greater than the critical angle, the light undergoes total internal reflection, reflecting back inside the droplet. 3. Refraction & Emergence: The reflected light then undergoes a second refraction as it exits the droplet and enters the air, further separating the colors and directing them towards the observer's eye. Each color emerges at a slightly different angle, creating the arc of the rainbow.

Q24: Explain the relationship between the speed of light in a medium and its refractive index.

Answer: The refractive index (n) of a medium is inversely proportional to the speed of light (v) in that medium. The relationship is given by the formula: n = c/v, where 'c' is the speed of light in vacuum (a constant). This means: If the speed of light in a medium is lower, its refractive index will be higher (optically denser medium). If the speed of light in a medium is higher, its refractive index will be lower (optically rarer medium). This inverse relationship is the fundamental reason why light bends when it crosses the boundary between two media with different optical densities.

Q25: Differentiate between 'real depth' and 'apparent depth' and explain why they differ.

Answer: Real Depth: The actual physical depth of an object submerged in a denser medium (e.g., the true depth of a swimming pool). Apparent Depth: The perceived depth of an object submerged in a denser medium, which appears shallower than its real depth when viewed from a rarer medium (e.g., air). They differ due to refraction of light. When light rays from the submerged object travel from the denser medium (water) to the rarer medium (air) and enter our eyes, they bend away from the normal. Our brain, accustomed to light traveling in straight lines, traces these refracted rays back to a point that is shallower than the actual object's position, creating the illusion of reduced depth.

Q26: What are the factors that affect the critical angle for a pair of media?

Answer: The critical angle for a pair of media is affected by: 1. Refractive Indices of the Two Media: The critical angle depends directly on the ratio of the refractive indices of the denser and rarer media. Specifically, sin C = n_rarer / n_denser. A larger difference in refractive indices leads to a smaller critical angle. 2. Color (Wavelength) of Light: Since the refractive index of a medium varies slightly with the wavelength of light (dispersion), the critical angle will also be slightly different for different colors of light. For example, violet light (higher refractive index) will have a smaller critical angle than red light (lower refractive index) in the same medium.

Q27: How is refraction utilized in optical instruments like telescopes and microscopes?

Answer: Refraction is the fundamental principle behind the operation of lenses, which are the core components of telescopes and microscopes: Telescopes: Use lenses (or mirrors) to gather light from distant objects and refract it to form a magnified image, allowing us to see celestial bodies. The objective lens forms a real, inverted image, which is then further magnified by the eyepiece. Microscopes: Use a combination of lenses to magnify very small objects. The objective lens forms a magnified real image of the tiny object, which is then further magnified by the eyepiece to produce a highly enlarged virtual image. In both cases, the precise bending of light by lenses allows for the manipulation of light rays to form magnified and observable images.

Q28: Why does a diamond sparkle more than a glass piece of the same shape?

Answer: A diamond sparkles more than a glass piece of the same shape due to two main optical properties: 1. Higher Refractive Index: Diamond has a much higher refractive index (approx. 2.42) compared to glass (approx. 1.5). This means light bends significantly more when entering a diamond. 2. Smaller Critical Angle: Due to its higher refractive index, diamond has a very small critical angle (approx. 24.4°). This small critical angle means that light entering a diamond is much more likely to undergo total internal reflection multiple times within the diamond's facets before exiting. This multiple internal reflections and subsequent dispersion of light into its constituent colors (fire) contribute to its extraordinary brilliance and sparkle, making it appear more luminous than glass.

Q29: Differentiate between reflection and refraction of light.

Answer: Reflection: Occurs when light rays bounce back from a surface. Light stays in the same medium. Angle of incidence equals angle of reflection (Law of Reflection). Causes images in mirrors. Refraction: Occurs when light rays bend as they pass through a boundary between two different transparent media. Light enters a new medium. Angle of incidence and angle of refraction are related by Snell's Law (sin i / sin r = constant). Causes objects to appear bent in water, lenses to form images, and prisms to disperse light.

References

1. MSBSHSE Class 10 Science and Technology Textbook Part 1 (2021-22 English) - Chapter 6: Refraction of Light
2. Maharashtra State Board 10th Standard Science Syllabus 2025-26
3. Balbharati Science and Technology Part 1 Textbook
4. Shaalaa.com Balbharati solutions for Science and Technology 1