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Chapter 10: THE HUMAN EYE AND THE COLOURFUL WORLD
1. Introduction
The human eye is one of the most precious sense organs that enables us to see the beautiful world around us. It works like a natural camera that captures light from objects and forms an image on a light-sensitive screen inside the eye called the retina.
2. Structure of the Human Eye
The main parts of the human eye and their functions are as follows:
Cornea:
- The transparent outermost layer of the eye.
- It allows light to enter the eye and bends (refracts) most of the light rays.
Aqueous Humour:
- A clear fluid found between the cornea and the lens.
- It nourishes the cornea and lens and maintains the shape of the eye.
Iris:
- A dark muscular structure behind the cornea.
- It controls the size of the pupil and hence the amount of light entering the eye.
Pupil:
- The small circular opening in the centre of the iris.
- It regulates the entry of light — it becomes larger in dim light and smaller in bright light.
Eye Lens:
- A transparent, flexible, convex lens made of living tissue.
- It focuses light rays on the retina to form a clear image.
Ciliary Muscles:
- These muscles hold the lens and control its curvature (shape).
- By relaxing or contracting, they change the focal length of the lens for focusing on near or distant objects.
Vitreous Humour:
- A jelly-like transparent fluid between the lens and retina.
- It helps maintain the spherical shape of the eye.
Retina:
- The light-sensitive inner surface at the back of the eye.
- It contains rods (for dim light) and cones (for color vision).
- The image formed on the retina is real and inverted.
Optic Nerve:
- It carries the visual information from the retina to the brain.
- The brain interprets these signals and we perceive the object in an upright position.
Blind Spot:
- The point on the retina where the optic nerve leaves the eye.
- No rods or cones are present here, hence no image is formed at this spot.
3. Working of the Human Eye
- Light rays from an object enter the eye through the cornea.
- They are refracted by the cornea and aqueous humour, then further by the lens.
- The lens focuses the light rays onto the retina, forming a real, inverted image.
- The retina converts the light energy into electrical signals which travel to the brain through the optic nerve.
- The brain processes these signals and interprets the image as upright.
4. Power of Accommodation
The ability of the eye lens to adjust its focal length to see objects clearly at different distances is called the power of accommodation.
- When viewing distant objects, ciliary muscles relax → lens becomes thinner → focal length increases.
- When viewing near objects, ciliary muscles contract → lens becomes thicker → focal length decreases.
The average power of accommodation of a healthy human eye is about 4 dioptres.
5. Range of Vision
- Least distance of distinct vision (Near Point):
The minimum distance at which an object can be seen clearly and distinctly is about 25 cm for a normal eye. - Far Point:
The maximum distance at which an object can be seen clearly is infinity.
Thus, the range of vision of a normal human eye is from 25 cm to infinity.
6. Defects of Vision and Their Correction
(a) Myopia (Near-sightedness)
- A person can see near objects clearly but distant objects appear blurred.
- The image of a distant object forms in front of the retina.
Cause:
- Eye ball is too long
- Lens has too much converging power.
Correction:
- Use a concave lens of suitable power which diverges light rays before they enter the eye.
- This allows the image to form on the retina.
(b) Hypermetropia (Far-sightedness)
- A person can see distant objects clearly but near objects appear blurred.
- The image of a nearby object forms behind the retina.
Cause:
- Eyeball is too short
- Lens has insufficient converging power.
Correction:
- Use a convex lens of suitable power to converge the light rays before they enter the eye.
(c) Presbyopia
- Occurs in old age due to loss of flexibility of the ciliary muscles and the lens.
- Person cannot see near objects clearly.
- Sometimes accompanied by myopia.
Correction:
- Use bifocal lenses – upper part concave (for distance) and lower part convex (for reading).
(d) Astigmatism
- Eye cannot focus light rays equally in all directions; image becomes distorted or blurred.
Correction: Use of cylindrical lenses.
7. Refraction of Light through the Atmosphere (Atmospheric Refraction)
The earth’s atmosphere has layers of air with varying densities.
When light from the stars enters these layers, it bends continuously before reaching the observer.
Examples:
- Twinkling of stars:
- Light from a star undergoes atmospheric refraction due to different air densities.
- The apparent position of the star changes continuously, so stars appear to twinkle.
- Apparent position of stars:
- The actual position of a star is slightly different from its apparent position because light bends while entering the atmosphere.
- Early sunrise and delayed sunset:
- The sun appears to rise about 2 minutes earlier and set 2 minutes later than the actual time because of atmospheric refraction.
- Light from the sun reaches us even when it is below the horizon.
8. Dispersion of Light
Dispersion is the splitting of white light into its seven constituent colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red – VIBGYOR) when it passes through a prism.
Cause:
Each color of light has a different wavelength and therefore bends by a different amount on refraction.
Red light bends the least, while violet bends the most.
9. Formation of Rainbow
- A rainbow is formed due to dispersion, refraction, and reflection of sunlight by water droplets in the atmosphere.
- Each water droplet acts like a small prism.
- Sunlight enters the droplet and splits into its constituent colours.
- The light is reflected inside the droplet and again refracted while coming out.
- The seven colours combine to form a circular arc of a rainbow.
Order of colours (top to bottom):
Red, Orange, Yellow, Green, Blue, Indigo, Violet.
10. Scattering of Light
Scattering of light is the process of spreading of light in different directions when it hits small particles in its path.
Key Factors Affecting Scattering:
- Shorter wavelength → greater scattering.
- Longer wavelength → lesser scattering.
Hence, blue light (shorter wavelength) scatters more than red light (longer wavelength).
11. Tyndall Effect
The scattering of light by particles in a colloid or in very fine suspension is called the Tyndall effect.
Examples:
- Sunlight entering a dark room through small holes appears as a beam.
- Headlights in fog or dust appear as visible beams.
Conditions for Tyndall Effect:
- The size of the particles must be smaller than the wavelength of visible light.
- The refractive index of the particles and the medium should differ.
12. Why is the Sky Blue?
- The atmosphere contains small particles that scatter the shorter wavelength (blue light) more effectively.
- Hence, the sky appears blue to us during daytime.
If the earth had no atmosphere, the sky would appear black.
13. Why are Sunrises and Sunsets Red?
- During sunrise and sunset, sunlight travels through a thicker layer of atmosphere.
- Most of the blue and green light is scattered away.
- Only red and orange (longer wavelengths) reach our eyes.
- Hence, the sun and the surrounding sky appear reddish at sunrise and sunset.
14. Why Does the Sky Appear Dark from Space?
- There is no atmosphere in space to scatter sunlight.
- Therefore, the sky appears completely dark to astronauts.
15. Summary of Phenomena
Phenomenon | Cause | Example |
Reflection | Bouncing back of light | Image in mirror |
Refraction | Bending of light | Pencil in water appears bent |
Dispersion | Splitting of light | Prism experiment |
Scattering | Spreading of light | Blue color of sky |
Tyndall Effect | Scattering by particles | Light beam in fog |
Atmospheric Refraction | Bending of light in air layers | Twinkling of stars |
16. Important Formulae
- Lens Formula: 1/f = 1/v – 1/u
- Magnification: m = h’/h = v/u
- Power of Lens: P = 1/f (f in meters)
17. Applications in Daily Life
- Glasses or contact lenses correct eye defects.
- Cameras and projectors use the principle of refraction.
- Rainbows and mirages are results of light dispersion and refraction.
Blue sky and red sunsets are due to scattering.
18. Conclusion
The human eye is a complex optical device that allows us to see the colourful world. Understanding reflection, refraction, dispersion, and scattering helps us explain natural phenomena like rainbows, blue sky, and twinkling stars, making physics more meaningful in everyday life.
✅ Oswaal Class 10 SST — Topper's Choice
✅ Together With SST — Best for Practice
✅ Oswaal Science Class 10 — Chapter Wise
✅ RD Sharma Maths — Most Popular
✅ Oswaal Class 10 SST — Topper's Choice
✅ Together With SST — Best for Practice
✅ Oswaal Science Class 10 — Chapter Wise
✅ RD Sharma Maths — Most Popular