Navigating through the CBSE Class 10 Science curriculum requires an in-depth understanding of ocular physiology, accommodation mechanisms, vision defects, prism dispersion, atmospheric refraction, and Rayleigh scattering. Chapter 10 of Class 10 Physics, “The Human Eye and the Colourful World”, explores how the human eye acts as a living optical instrument and investigates the physics behind natural optical phenomena. It details the causes and optical ray diagram corrections for Myopia, Hypermetropia, and Presbyopia; explains the dispersion of white light through a triangular glass prism into the VIBGYOR spectrum; and investigates why stars twinkle, why planets do not twinkle, why advanced sunrise and delayed sunset occur, and why the sky appears deep blue while sunrise/sunset appears crimson red. To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE evaluation standards.
Every question presented in the official NCERT textbook—ranging from all four in-text questions (Page 190) to the complete chapter-end exercises (Questions 1 to 13 on Pages 197–198)—has been solved with exhaustive detail. Numerical problems follow a step-by-step box format with explicit sign conventions, lens formulas, and optical power calculations. Key scoring terms, official CBSE board year tags, and comparative optical tables have been highlighted to ensure students secure maximum marks in their CBSE Board Examinations.
Master Formula & Concept Summary Tables
1. Vision Defects, Causes, and Optical Corrections
| Defect of Vision | Common Name | Nature of Defect (What is Blurred?) | Underlying Causes | Position of Image Formation | Corrective Optical Lens |
|---|---|---|---|---|---|
| Myopia | Near-sightedness | Distant objects are blurred; nearby objects are seen clearly. | (i) Excessive curvature of eye lens. (ii) Elongation of the eyeball. | Image is formed in front of the retina. | Concave Lens (Diverging Lens) of suitable focal length (P < 0). |
| Hypermetropia | Far-sightedness | Nearby objects are blurred; distant objects are seen clearly. | (i) Focal length of eye lens is too long. (ii) Eyeball has become too small. | Image is formed behind the retina. | Convex Lens (Converging Lens) of suitable focal length (P > 0). |
| Presbyopia | Old-Age Hypermetropia | Difficulty reading nearby text comfortably without glasses. | (i) Gradual weakening of ciliary muscles. (ii) Diminishing flexibility of crystalline lens. | Image of near objects falls behind retina. | Bifocal Lenses (Upper part = Concave for distance; Lower part = Convex for reading). |
| Cataract | Ocular Opacity | Complete or partial loss of vision due to hazy, milky lens. | Crystalline eye lens becomes milky and cloudy in old age. | Light rays are blocked and scattered before retina. | Surgical Removal of cloudy lens and replacement with an Intraocular Lens (IOL). |
2. Dispersion, Atmospheric Refraction, and Scattering Summary
| Optical Phenomenon | Primary Physical Principle | Physical Mechanism / Explanation | Key Real-World Examples |
|---|---|---|---|
| Dispersion of Light | Wavelength-dependent Refractive Index | Different colors of light travel with different speeds in glass. Violet bends the most (shortest wavelength); Red bends the least (longest wavelength). | Formation of Rainbow in the sky after rain; Glass prism spectrum (VIBGYOR). |
| Atmospheric Refraction | Continuous Refraction through air layers of varying optical density | Cold air is denser than warm air. As light descends through atmosphere, it bends continuously toward the normal. | (i) Twinkling of Stars. (ii) Apparent flattening of Sun’s disc. (iii) Advance Sunrise (2 min) and Delayed Sunset (2 min). |
| Scattering of Light | Rayleigh’s Law of Scattering (Scattering Intensity ∝ 1 / λ⁴) | Very fine air molecules scatter shorter wavelengths (Blue/Violet) far more strongly than longer wavelengths (Red). | (i) Clear sky appears Blue. (ii) Sun appears Reddish at Sunrise & Sunset. (iii) Danger signal lights are Red. |
NCERT In-Text Questions (Page No. 190)
Question 1 What is meant by power of accommodation of the eye? [CBSE 2024, 2020]
Answer: The power of accommodation is the biological ability of the human crystalline eye-lens to adjust its focal length automatically—through the contraction and relaxation of the ciliary muscles—so that sharp and clear images of objects situated at varying distances (from 25 cm to infinity) are consistently focused onto the retina.
Question 2 A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision? [CBSE 2024, 2022]
Answer:
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NUMERICAL SOLUTION (CORRECTION OF MYOPIA):
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GIVEN DATA:
• Far point of myopic eye (d) = 1.2 m
• To see distant objects at infinity clearly:
- Object distance (u) = -∞ (Infinity)
- Image distance (v) = -1.2 m (Virtual image formed at the person's far point)
CALCULATION (LENS FORMULA):
Formula: 1/f = 1/v - 1/u
1/f = 1/(-1.2) - 1/(-∞)
1/f = -1 / 1.2 - 0
f = -1.2 m (or -120 cm)
POWER OF CORRECTIVE LENS (P):
Formula: P = 1 / f (in m)
P = 1 / (-1.2) = -10 / 12 = -0.83 D
FINAL ANSWER:
• Type of Lens : Concave Lens (Diverging Lens).
• Focal Length : -1.2 m (-120 cm).
• Optical Power: -0.83 Dioptres (-0.83 D).
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Question 3 What is the far point and near point of the human eye with normal vision? [CBSE 2023, 2020]
Answer:
- Near Point (Least Distance of Distinct Vision, D): The minimum distance at which an object can be seen most distinctly without any ocular strain is 25 cm for a normal young adult eye.
- Far Point: The farthest point up to which the eye can see objects clearly is Infinity (∞) for a normal eye.
Question 4 A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected? [CBSE 2024, 2023]
Answer:
- Defect of Vision: The student is suffering from Myopia (Near-sightedness) because they can read their desk book clearly but cannot focus on distant objects like the classroom blackboard.
- Optical Correction: This defect is corrected by prescribing spectacles fitted with a Concave Lens (Diverging Lens) of suitable focal length and power, which diverges the incoming parallel light rays so that they focus precisely onto the retina.
NCERT Chapter-End Exercises (Page No. 197-198)
Question 1 The human eye can focus objects at different distances by adjusting the focal length of the eye-lens. This is due to: (a) presbyopia (b) accommodation (c) near-sightedness (d) early-sightedness
Answer: (b) accommodation Explanation: Accommodation is the dynamic adjustment of the eye lens’s curvature and focal length by the ciliary muscles to focus objects from 25 cm to infinity onto the retina.
Question 2 The human eye forms the image of an object at its: (a) cornea (b) iris (c) pupil (d) retina
Answer: (d) retina Explanation: The retina is the light-sensitive inner screen of the eye containing photoreceptor rods and cones that generate electrical signals transmitted to the brain via the optic nerve.
Question 3 The least distance of distinct vision for a young adult with normal vision is about: (a) 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 m
Answer: (c) 25 cm Explanation: The standard least distance of distinct vision (near point) for a healthy young adult without eye strain is 25 cm.
Question 4 The change in focal length of an eye-lens is caused by the action of the: (a) pupil (b) retina (c) ciliary muscles (d) iris
Answer: (c) ciliary muscles Explanation: When ciliary muscles relax, the lens becomes thin (focal length increases for distant vision); when they contract, the lens thickens (focal length decreases for near vision).
Question 5 A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision? [CBSE 2024, 2022]
Answer:
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NUMERICAL SOLUTION (BIFOCAL LENS FOCAL LENGTHS):
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(i) For Distant Vision:
Given Power (P₁) = -5.5 D
Formula: f₁ = 1 / P₁ (in metres)
f₁ = 1 / (-5.5) = -10 / 55 = -2 / 11 m
f₁ = -0.1818 m = -18.18 cm
Type: Concave Lens of focal length -18.18 cm.
(ii) For Near Vision:
Given Power (P₂) = +1.5 D
Formula: f₂ = 1 / P₂ (in metres)
f₂ = 1 / (+1.5) = +10 / 15 = +2 / 3 m
f₂ = +0.667 m = +66.67 cm
Type: Convex Lens of focal length +66.67 cm.
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Question 6 The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem? [CBSE 2024, 2020]
Answer:
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NUMERICAL SOLUTION (MYOPIA CORRECTION):
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GIVEN DATA:
• Object distance (u) = -∞ (Distant object at infinity)
• Image distance (v) = -80 cm = -0.8 m (Far point of myopic eye)
STEP 1: Finding Focal Length (f)
Formula: 1/f = 1/v - 1/u
1/f = 1/(-0.8) - 1/(-∞)
1/f = -1 / 0.8 - 0
f = -0.8 m = -80 cm
STEP 2: Calculating Power of Lens (P)
Formula: P = 1 / f (in m)
P = 1 / (-0.8) = -10 / 8 = -1.25 D
FINAL ANSWER:
• Nature of Corrective Lens : Concave Lens (Diverging Lens).
• Focal Length (f) : -80 cm (-0.8 m).
• Optical Power (P) : -1.25 Dioptres (-1.25 D).
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Question 7 Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm. [CBSE 2024, 2023]
Answer:
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NUMERICAL SOLUTION (HYPERMETROPIA CORRECTION):
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GIVEN DATA:
• Object distance (u) = -25 cm = -0.25 m (Normal near point)
• Image distance (v) = -1 m = -100 cm (Defective near point)
STEP 1: Finding Focal Length (f)
Formula: 1/f = 1/v - 1/u
1/f = 1/(-100) - 1/(-25)
1/f = -1/100 + 1/25
1/f = (-1 + 4) / 100 = +3 / 100
f = +100 / 3 cm = +1/3 m = +0.333 m
STEP 2: Calculating Power of Lens (P)
Formula: P = 1 / f (in m)
P = 1 / (+1/3)
P = +3.0 D
FINAL ANSWER:
• Nature of Lens : Convex Lens (Converging Lens).
• Power of Lens : +3.0 Dioptres (+3.0 D).
RAY DIAGRAM EXPLANATION:
1. Hypermetropic Eye: Light rays from a near object (at 25 cm) converge behind
the retina.
2. Corrected Eye: A convex lens placed in front of the eye provides additional
convergence, forming a virtual image at the defective near point (1 m) so
that final rays focus sharply on the retina.
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Question 8 Why is a normal eye not able to see clearly the objects placed closer than 25 cm? [CBSE 2024, 2020]
Answer: A normal human eye cannot focus on objects closer than 25 cm because the ciliary muscles have a maximum limit of contraction:
- The crystalline eye-lens cannot be curved or thickened beyond a certain minimum radius of curvature.
- Consequently, the focal length of the eye-lens cannot be reduced below a specific minimum value.
- If an object is placed closer than 25 cm, the converging power is insufficient, and the light rays focus behind the retina, resulting in a blurred image and severe eye strain.
Question 9 What happens to the image distance in the eye when we increase the distance of an object from the eye? [CBSE 2023]
Answer: The image distance inside the eye remains completely constant and unchanged (equal to the fixed distance between the eye-lens and the retina, approximately 2.5 cm).
- Mechanism: When the object distance increases, the ciliary muscles relax, making the eye-lens thinner and increasing its focal length (
f). This adjustment ensures that the image is always formed precisely on the retina.
Question 10 Why do stars twinkle? [CBSE 2024, 2020]
Answer: The twinkling of stars is caused by Atmospheric Refraction of starlight:
================================================================================ STEP-BY-STEP MECHANISM OF TWINKLING OF STARS: -------------------------------------------------------------------------------- 1. Point-Sized Light Source: Stars are located extremely far away from Earth, acting as point sources of light. 2. Varying Optical Density: Earth's atmosphere consists of continuously moving layers of air with varying temperatures and optical refractive indices. 3. Continuous Refraction: As starlight enters the Earth's atmosphere, it undergoes continuous refraction, causing the path of the light ray to fluctuate slightly. 4. Fluctuation in Intensity: The apparent position of the star shifts continuously, and the amount of starlight entering the eye fluctuates—appearing bright one moment and faint the next. This rapid flickering is perceived as twinkling. ================================================================================
Question 11 Explain why the planets do not twinkle. [CBSE 2024, 2022]
Answer: Planets do not twinkle for two fundamental physical reasons:
- Extended Optical Sources: Planets are much closer to the Earth than distant stars and appear as extended optical discs rather than point sources.
- Nullification Effect: A planet can be considered a collection of a large number of point-sized light sources. While light from some points brightens due to atmospheric refraction, light from other points dims simultaneously. The total average variation in light entering the eye averages out to zero, nullifying the twinkling effect.
Question 12 Why does the Sun appear reddish early in the morning? [CBSE 2024, 2020]
Answer: The reddish appearance of the Sun at sunrise (and sunset) is caused by the Scattering of Light (Rayleigh Scattering):
- Near the horizon, sunlight must travel through a much thicker layer of the atmosphere and a greater distance to reach our eyes.
- Shorter wavelengths of light (blue and violet) are scattered away in all directions by fine atmospheric air particles (
Scattering Intensity ∝ 1 / λ⁴). - The longer wavelengths of light (red and orange) undergo the least scattering and penetrate the atmosphere to reach our eyes, making the rising Sun appear crimson red.
Question 13 Why does the sky appear dark instead of blue to an astronaut? [CBSE 2024, 2023]
Answer: The sky appears dark (black) to an astronaut in outer space because there is no atmosphere in space to cause the scattering of sunlight.
In the complete absence of air molecules and particulate matter, sunlight travels in straight lines without being scattered in different directions. Since no scattered light enters the astronaut’s eyes, the surrounding sky appears completely black.
Frequently Asked Questions (FAQs) – Class 10 Physics Chapter 10
Question 1: What is the function of the Iris and Pupil in the human eye? [CBSE 2024] Answer: The Iris is a dark muscular diaphragm that controls the size of the pupil. The Pupil acts as an aperture that regulates and controls the amount of light entering the eye (constricting in bright light and dilating in dim light).
Question 2: What is the role of the Cornea? [CBSE 2023] Answer: The cornea is the transparent, curved front membrane of the eyeball that provides roughly 80% of the total optical refraction for light rays entering the eye.
Question 3: What is the function of Rods and Cones on the retina? [CBSE 2024] Answer:
- Rods: Photoreceptors that are highly sensitive to low light intensity (dim light/night vision).
- Cones: Photoreceptors sensitive to bright light and responsible for color vision.
Question 4: What is Presbyopia, and how is it corrected? [CBSE 2024, 2022] Answer: Presbyopia (old-age hypermetropia) is the age-related loss of accommodation due to the weakening of ciliary muscles and hardening of the eye-lens. It is corrected using bifocal lenses (concave upper part for distance, convex lower part for reading).
Question 5: What is the angle of deviation (D) in a glass prism? [CBSE 2023] Answer: The angle of deviation is the angle between the extended path of the original incident light ray and the path of the emergent light ray emerging from a triangular glass prism.
Question 6: Why does white light disperse when passing through a prism? [CBSE 2024] Answer: White light disperses because different constituent colors of light have different wavelengths and travel at different speeds in glass, causing each color to refract at a different angle.
Question 7: How is a natural rainbow formed in the sky? [CBSE 2024, 2020] Answer: A rainbow is formed by the dispersion of sunlight by tiny suspended raindrops acting as microscopic prisms through three successive steps: Refraction with Dispersion → Internal Reflection → Refraction on emerging.
Question 8: Why do danger signals and traffic stoplights use red color? [CBSE 2024] Answer: Red light has the longest visible wavelength in the spectrum and is scattered the least by atmospheric fog, smoke, and dust particles, allowing it to remain visible over long distances.
Question 9: What causes early sunrise and delayed sunset? By how much time? [CBSE 2023] Answer: Atmospheric refraction bends light rays coming from the Sun over the horizon, making the Sun visible 2 minutes before actual sunrise and 2 minutes after actual sunset, lengthening the day by 4 minutes.
Question 10: What is the Tyndall Effect? Give an everyday example. [CBSE 2024] Answer: The Tyndall effect is the optical scattering of a light beam by colloidal particles in its path, making the path of light visible (e.g., sunlight streaming through a dense forest canopy or a dusty room window).
Question 11: What is Astigmatism and how is it corrected? [CBSE 2022] Answer: Astigmatism is a vision defect where horizontal and vertical planes cannot be focused simultaneously due to the irregular curvature of the cornea. It is corrected using Cylindrical Lenses.
Question 12: Why does the sky appear deep blue on a clear sunny day? [CBSE 2024] Answer: Fine nitrogen and oxygen molecules in the upper atmosphere have sizes smaller than the wavelength of visible light. According to Rayleigh’s law, they scatter short-wavelength blue light much more strongly than red light.
Question 13: Why does the Sun look white when it is directly overhead at noon? [CBSE 2023] Answer: At noon, sunlight travels the shortest distance through the atmosphere. Very little blue and violet light is scattered away, so all colors reach the observer in roughly equal proportions, making the Sun appear white.
Question 14: What is the persistence of vision in the human eye? [BOARD EXAM FAVORITE] Answer: Persistence of vision is the biological phenomenon where an optical image impression lingers on the human retina for approximately 1/16th of a second after the object is removed, enabling the illusion of motion in cinema.
Question 15: Can a person with Myopia also develop Presbyopia in old age? [CBSE 2024] Answer: Yes. A myopic individual will retain near-sightedness while the aging ciliary muscles lose accommodation, requiring bifocal lenses containing both concave and convex components.
Mastering the NCERT Solutions for Class 10 Science Chapter 10, “The Human Eye and the Colourful World”, equips students with the ocular ray diagrams, lens power calculations, and atmospheric refraction concepts required for the CBSE Board Examination. Review the eye defect correction numericals, the dispersion spectrum, and the 15 board-level FAQs above to secure full marks in your physics evaluations.