Class X Physics - Chapter 2
Human Eye and Colourful World
10.1 The Human Eye
Human Eye: The human eye is a natural optical instrument that enables us to see the world around us. It works like a camera, forming real and inverted images on the retina.
Structure of Human Eye:
Human Eye
1. Cornea:
A transparent, bulging front surface of the eye that provides most of the refraction of light entering the eye. It has a fixed focal length.
2. Iris:
A dark, muscular diaphragm that controls the size of the pupil and hence the amount of light entering the eye. It gives color to the eye.
3. Pupil:
The opening in the center of the iris through which light enters the eye. Its size is controlled by the iris muscles.
4. Eye Lens (Crystalline Lens):
A transparent, biconvex lens made of a fibrous, jelly-like material. It provides fine adjustment of focal length required to focus objects at different distances on the retina.
5. Ciliary Muscles:
Muscles attached to the eye lens that can change the curvature and focal length of the lens by contracting or relaxing.
6. Retina:
The light-sensitive screen at the back of the eye containing millions of light-sensitive cells called rods and cones. It acts like the film in a camera.
7. Optic Nerve:
Carries visual information from the retina to the brain for processing and interpretation.
8. Blind Spot:
The point where the optic nerve leaves the eye. It has no light-sensitive cells, so no vision is possible at this point.
Functions of Different Parts:
- Cornea and Eye Lens: Converge light rays to form image on retina
- Iris: Controls amount of light entering the eye
- Retina: Contains photoreceptors (rods and cones)
- Rods: Sensitive to dim light (night vision)
- Cones: Sensitive to bright light and colors (day vision)
10.2 Power of Accommodation
Accommodation: The ability of the eye lens to adjust its focal length to focus objects at different distances clearly on the retina is called accommodation.
Mechanism of Accommodation:
- For distant objects: Ciliary muscles relax → lens becomes thinner → focal length increases
- For near objects: Ciliary muscles contract → lens becomes thicker → focal length decreases
Far Point: The farthest point up to which the eye can see objects clearly. For a normal eye, it is at infinity.
Near Point: The nearest point up to which the eye can see objects clearly. For a normal eye, it is at 25 cm from the eye.
Range of Vision for Normal Eye:
Near Point = 25 cm
Far Point = ∞ (infinity)
Example: When you shift your gaze from a distant mountain to reading a book, your eye lens automatically adjusts its curvature to maintain clear vision. This automatic adjustment is accommodation.
10.3 Defects of Vision and Their Correction
1. Myopia (Short-sightedness or Near-sightedness)
Myopia: A defect of vision in which a person can see nearby objects clearly but cannot see distant objects clearly.
Causes of Myopia:
- Excessive curvature of the cornea
- Elongation of the eyeball
- Image forms in front of the retina
Correction of Myopia:
Correction: Use of Concave Lens (Diverging Lens)
The concave lens diverges the light rays before they enter the eye
2. Hypermetropia (Long-sightedness or Far-sightedness)
Hypermetropia: A defect of vision in which a person can see distant objects clearly but cannot see nearby objects clearly.
Causes of Hypermetropia:
- Flattening of the cornea
- Shortening of the eyeball
- Image forms behind the retina
- Weakening of ciliary muscles
Correction of Hypermetropia:
Correction: Use of Convex Lens (Converging Lens)
The convex lens converges the light rays before they enter the eye
3. Presbyopia
Presbyopia: A defect of vision that occurs with aging in which the eye gradually loses its power of accommodation due to weakening of ciliary muscles and decreasing flexibility of the eye lens.
Correction of Presbyopia:
- Bifocal Lenses: Upper part has concave lens for distant vision, lower part has convex lens for near vision
- Progressive Lenses: Gradual change in power from top to bottom
| Defect |
Can See Clearly |
Cannot See Clearly |
Cause |
Correction |
| Myopia |
Near objects |
Distant objects |
Image forms in front of retina |
Concave lens |
| Hypermetropia |
Distant objects |
Near objects |
Image forms behind retina |
Convex lens |
| Presbyopia |
Neither near nor far |
Both near and far |
Loss of accommodation |
Bifocal lens |
10.4 Refraction of Light through a Prism
Prism: A transparent optical device with flat, polished surfaces that refract light. A triangular prism has two triangular bases and three rectangular lateral surfaces.
Important Terms:
- Angle of Prism (A): Angle between two refracting surfaces
- Angle of Deviation (δ): Angle between incident ray and emergent ray
- Angle of Incidence (i₁): Angle between incident ray and normal at first surface
- Angle of Emergence (i₂): Angle between emergent ray and normal at second surface
For a Prism:
A = r₁ + r₂
δ = i₁ + i₂ - A
Where r₁ and r₂ are angles of refraction at two surfaces
Example: When white light passes through a prism, it splits into seven colors (VIBGYOR) because different colors have different refractive indices and bend by different amounts.
10.5 Dispersion of White Light
Dispersion: The phenomenon of splitting of white light into its constituent colors when passed through a prism is called dispersion of light.
Spectrum of White Light:
VIBGYOR: Violet → Indigo → Blue → Green → Yellow → Orange → Red
Cause of Dispersion:
- Different colors have different wavelengths
- Different wavelengths have different refractive indices in the same medium
- Red light has maximum wavelength, minimum refractive index, minimum deviation
- Violet light has minimum wavelength, maximum refractive index, maximum deviation
Order of Deviation:
δviolet > δindigo > δblue > δgreen > δyellow > δorange > δred
Recombination: When the dispersed colors are passed through another identical prism in inverted position, they recombine to form white light.
10.6 Rainbow Formation
Rainbow: A natural phenomenon showing dispersion of sunlight by tiny water droplets suspended in the atmosphere, forming a spectrum of seven colors in the sky.
Conditions for Rainbow Formation:
- Sun should be behind the observer
- Water droplets should be in front of the observer
- Sun should not be too high in the sky
Formation Process:
- Refraction: Sunlight enters the water droplet and disperses into colors
- Total Internal Reflection: Light reflects from the back of the droplet
- Refraction: Light exits the droplet, further separating the colors
Types of Rainbow:
- Primary Rainbow: Formed by one internal reflection, violet on inside
- Secondary Rainbow: Formed by two internal reflections, violet on outside (fainter)
10.7 Atmospheric Refraction
Atmospheric Refraction: The refraction of light by the Earth's atmosphere is called atmospheric refraction. It occurs due to variation in refractive index of air at different heights.
Causes:
- Air density decreases with altitude
- Refractive index decreases with altitude
- Light rays bend towards the denser medium (downward)
Phenomena Due to Atmospheric Refraction:
1. Twinkling of Stars:
- Stars appear to twinkle due to atmospheric refraction
- Light from stars continuously bends as it passes through different air layers
- The apparent position of stars keeps changing
- Planets don't twinkle as they are much closer and appear as extended sources
2. Advanced Sunrise and Delayed Sunset:
- Sun appears to rise about 2 minutes before actual sunrise
- Sun appears to set about 2 minutes after actual sunset
- Due to refraction, sun appears above horizon even when it's below
Time Difference:
Advanced sunrise = 2 minutes
Delayed sunset = 2 minutes
Total increase in day length = 4 minutes
10.8 Scattering of Light
Scattering of Light: The phenomenon in which light rays deviate from their straight path when they interact with particles in their path is called scattering of light.
Rayleigh Scattering:
When the size of scattering particles is much smaller than the wavelength of light, the intensity of scattered light is inversely proportional to the fourth power of wavelength.
Rayleigh's Law:
I ∝ 1/λ⁴
Where I = Intensity of scattered light, λ = Wavelength
Applications of Scattering:
1. Blue Color of Sky:
- Blue light has shorter wavelength than red light
- Blue light scatters more than red light (∝ 1/λ⁴)
- Our eyes receive more scattered blue light, making sky appear blue
- At higher altitudes, sky appears darker as there's less atmosphere to scatter light
2. Red Color of Sun at Sunrise and Sunset:
- During sunrise/sunset, sunlight travels longer path through atmosphere
- Blue light gets scattered away by atmospheric particles
- Red light, being least scattered, reaches our eyes
- Sun appears red-orange during these times
3. Danger Signals are Red:
- Red light has longest wavelength
- Red light scatters least and travels farthest
- Can be seen from maximum distance even in foggy conditions
Example: When you look at the sky through sunglasses, it appears less blue because the glasses reduce the intensity of scattered blue light reaching your eyes.
10.9 Tyndall Effect
Tyndall Effect: The phenomenon of scattering of light by colloidal particles suspended in a medium is called the Tyndall effect.
Conditions for Tyndall Effect:
- Size of particles should be comparable to wavelength of light
- Particles should be suspended in a transparent medium
- There should be difference in refractive indices of particles and medium
Examples of Tyndall Effect:
- Light beam visible in a dusty room
- Light beam from car headlights in fog
- Light beam in cinema halls
- Blue color of smoke from motorcycles
- Milk appears blue when diluted with water
Note: True solutions do not show Tyndall effect because particle size is too small. Colloidal solutions show Tyndall effect.
Practice Questions
1. The change in focal length of eye lens is caused by:
(A) Pupil
(B) Retina
(C) Ciliary muscles
(D) Iris
Answer: (C)
2. The near point of a normal human eye is:
(A) 25 m
(B) 25 cm
(C) 2.5 cm
(D) Infinity
Answer: (B)
3. A person suffering from myopia uses:
(A) Convex lens
(B) Concave lens
(C) Bifocal lens
(D) No lens
Answer: (B)
4. The splitting of white light into seven colors is called:
(A) Reflection
(B) Refraction
(C) Dispersion
(D) Scattering
Answer: (C)
5. The color of light that deviates least through a prism is:
(A) Violet
(B) Blue
(C) Yellow
(D) Red
Answer: (D)
6. Stars twinkle due to:
(A) Atmospheric reflection
(B) Atmospheric refraction
(C) Atmospheric scattering
(D) Atmospheric absorption
Answer: (B)
7. The sky appears blue due to:
(A) Reflection of light
(B) Refraction of light
(C) Scattering of light
(D) Absorption of light
Answer: (C)
8. Danger signals are red because:
(A) Red light travels fastest
(B) Red light is scattered least
(C) Red light has maximum energy
(D) Red light is most visible
Answer: (B)
9. The phenomenon responsible for the blue color of the sky is:
(A) Tyndall effect
(B) Rayleigh scattering
(C) Mie scattering
(D) Total internal reflection
Answer: (B)
10. A person with presbyopia should use:
(A) Convex lens
(B) Concave lens
(C) Bifocal lens
(D) No lens
Answer: (C)
11. The part of the eye that controls the amount of light entering is:
(A) Cornea
(B) Iris
(C) Pupil
(D) Retina
Answer: (B)
12. Rainbow is formed due to:
(A) Refraction only
(B) Reflection only
(C) Dispersion and total internal reflection
(D) Scattering of light
Answer: (C)
13. The intensity of scattered light is inversely proportional to:
(A) λ
(B) λ²
(C) λ³
(D) λ⁴
Answer: (D)
14. The Tyndall effect is observed when:
(A) Light passes through a true solution
(B) Light passes through a colloidal solution
(C) Light reflects from a mirror
(D) Light refracts through a lens
Answer: (B)
15. Advanced sunrise and delayed sunset are due to:
(A) Scattering of light
(B) Atmospheric refraction
(C) Dispersion of light
(D) Total internal reflection
Answer: (B)
16. The retina of the eye is comparable to which part of a camera?
(A) Lens
(B) Shutter
(C) Film
(D) Aperture
Answer: (C)
17. Which defect of vision is corrected by using a cylindrical lens?
(A) Myopia
(B) Hypermetropia
(C) Astigmatism
(D) Presbyopia
Answer: (C)
18. The light sensitive cells in the retina are:
(A) Rods and cones
(B) Rods and cylinders
(C) Cones and spheres
(D) Cubes and cones
Answer: (A)
19. At noon, the sun appears white because:
(A) Light travels shorter distance through atmosphere
(B) Light travels longer distance through atmosphere
(C) There is no atmospheric refraction
(D) There is no scattering
Answer: (A)
20. The phenomenon of light responsible for the working of optical fibers is:
(A) Refraction
(B) Scattering
(C) Total internal reflection
(D) Dispersion
Answer: (C)
10.10 Additional Phenomena and Applications
Astigmatism:
Astigmatism: A defect of vision in which the eye cannot focus light rays from all directions equally well on the retina due to irregular curvature of the cornea or lens.
Correction of Astigmatism:
- Corrected using cylindrical lenses
- The lens has different curvatures in different planes
- Compensates for the irregular curvature of the cornea
Color Blindness:
Color Blindness: A genetic defect in which a person cannot distinguish between certain colors, most commonly red and green, due to absence or malfunction of cone cells.
Night Blindness:
Night Blindness (Nyctalopia): A condition in which a person has poor vision in dim light due to deficiency of vitamin A, affecting the rod cells in the retina.
10.11 Important Numerical Problems
Problem 1: A person can read a book clearly when it is held at 50 cm from his eyes. Name the defect of vision he is suffering from. What kind of lens should he use to read the book when placed at 25 cm?
Solution:
Given: Near point = 50 cm (instead of normal 25 cm)
The person suffers from Hypermetropia
To read at 25 cm, we need:
Object distance (u) = -25 cm
Image distance (v) = -50 cm (at his near point)
Using lens formula: 1/f = 1/v - 1/u
1/f = 1/(-50) - 1/(-25) = -1/50 + 1/25 = (-1 + 2)/50 = 1/50
f = +50 cm = +0.5 m
Power P = 1/f = 1/0.5 = +2 D
Answer: Convex lens of power +2 D
Problem 2: A student sitting in the last row of the class cannot read the writing on the blackboard clearly but can read his book clearly. Name the defect and suggest correction.
Solution:
The student cannot see distant objects (blackboard) clearly but can see near objects (book) clearly.
Defect: Myopia (Short-sightedness)
Correction: Concave lens (diverging lens)
Problem 3: The far point of a myopic person is 200 cm. Calculate the power of the lens required to correct this defect.
Solution:
Given: Far point = 200 cm = 2 m
For correction, parallel rays (from infinity) should appear to come from the far point.
Object distance (u) = -∞
Image distance (v) = -200 cm = -2 m
Using lens formula: 1/f = 1/v - 1/u
1/f = 1/(-2) - 1/(-∞) = -1/2 - 0 = -1/2
f = -2 m
Power P = 1/f = 1/(-2) = -0.5 D
Answer: Concave lens of power -0.5 D
10.12 Summary of Key Concepts
Structure and Function:
- Human eye works like a camera with variable focal length
- Accommodation allows focusing at different distances
- Normal vision range: 25 cm to infinity
- Retina contains rods (dim light) and cones (bright light, color vision)
Vision Defects:
- Myopia: Near-sighted, corrected with concave lens
- Hypermetropia: Far-sighted, corrected with convex lens
- Presbyopia: Age-related, corrected with bifocal lens
- Astigmatism: Irregular focusing, corrected with cylindrical lens
Light Phenomena:
- Dispersion: Splitting of white light into spectrum
- Scattering: Deviation of light by particles (Rayleigh's law: I ∝ 1/λ⁴)
- Atmospheric refraction: Bending of light through atmosphere
- Tyndall effect: Scattering by colloidal particles
Natural Phenomena Explained:
- Blue sky: Scattering of blue light
- Red sun at sunrise/sunset: Blue light scattered away
- Twinkling stars: Atmospheric refraction
- Rainbow: Dispersion + total internal reflection in water droplets
- Advanced sunrise/delayed sunset: Atmospheric refraction
Important Formulas and Relations
For Prism:
A = r₁ + r₂
δ = i₁ + i₂ - A
Lens Formula:
1/f = 1/v - 1/u
Power of Lens:
P = 1/f (in meters)
Unit: Dioptre (D)
Magnification:
m = v/u
Rayleigh Scattering:
I ∝ 1/λ⁴
Refractive Index:
n = c/v = sin i/sin r
Practical Applications
Medical Applications:
- Corrective lenses for vision defects
- Contact lenses for better vision
- Surgical procedures like LASIK
- Cataract surgery with artificial lenses
Optical Instruments:
- Microscopes and telescopes using lens combinations
- Cameras with variable aperture (like iris)
- Endoscopes using optical fibers
- Spectacles and magnifying glasses
Atmospheric Applications:
- Weather prediction using light scattering
- Photography timing for golden hour effects
- Navigation using atmospheric phenomena
- Solar panel efficiency calculations
Experimental Activities
Activity 1: Observing Dispersion
Materials: Triangular glass prism, white light source, screen
Procedure: Allow white light to pass through prism and observe spectrum on screen
Observation: White light splits into VIBGYOR colors
Conclusion: Different colors have different refractive indices
Activity 2: Demonstrating Tyndall Effect
Materials: Beaker, water, milk, laser pointer
Procedure: Add few drops of milk to water and pass laser light through it
Observation: Light beam becomes visible in the milky water
Conclusion: Colloidal particles scatter light
Activity 3: Finding Focal Length of Eye Lens
Materials: Convex lens, object, screen, scale
Procedure: Form clear image of distant object and measure distance
Observation: Image forms at focal length from lens
Conclusion: For distant objects, image forms at focus
Common Misconceptions Clarified
Misconception 1: "Eye lens forms virtual images"
Fact: Eye lens always forms real, inverted images on retina. Brain interprets them as erect.
Misconception 2: "Myopic persons cannot see anything at distance"
Fact: They can see distant objects, but not clearly due to blurred vision.
Misconception 3: "Rainbow has only 7 colors"
Fact: Rainbow is a continuous spectrum; we conventionally divide it into 7 colors.
Misconception 4: "Planets don't twinkle because they don't emit light"
Fact: Planets don't twinkle because they appear as extended sources, not point sources.
Previous Years' Important Questions
Short Answer Questions (2-3 marks):
- Explain the structure and function of human eye.
- What is accommodation? How does it occur?
- Define myopia and hypermetropia with their corrections.
- Why do stars twinkle but planets do not?
- Explain the formation of rainbow.
- What is Tyndall effect? Give examples.
- Why is sky blue and sun red at sunrise/sunset?
Long Answer Questions (5 marks):
- Draw a neat diagram of human eye and explain its working.
- Describe the defects of vision and their correction with ray diagrams.
- Explain atmospheric refraction and its consequences.
- Describe dispersion of light through prism with diagram.
- Explain scattering of light with applications in daily life.
End of Chapter 2
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