Class 10 Science - Chapter 11: The Human Eye and the Colourful World (NCERT)
🖊Kishore Boruah, Teacher
Have you ever wondered how your eyes help you see the world in such vivid colors? Why do stars twinkle at night? Why does the sky look blue during the day but turns red at sunrise and sunset? All these fascinating phenomena are linked to the science of light and vision!
In this chapter, "The Human Eye and The Colourful World," you will explore the amazing structure of the human eye, how it works, and the different vision defects that can be corrected using lenses. You will also learn about incredible natural optical phenomena like the formation of rainbows, the twinkling of stars, and atmospheric refraction.
By the end of this chapter, you will understand:
✅ How the human eye works and adapts to different lighting conditions.
✅ What causes vision problems like myopia and hypermetropia, and how to correct them.
✅ How light behaves when it passes through different mediums, including prisms.
✅ Why the sky appears blue, the Sun looks red at sunrise and sunset, and planets do not twinkle like stars.
This topic You can translate to your own language
1. The Human Eye
The human eye is a natural optical device that helps us see the world. It works like a camera, focusing light to form images. The main parts of the eye and their functions are:
Parts of the Human Eye:
1. Cornea – The transparent outermost layer that protects the eye and helps in the refraction of light.
2. Iris – A colored part of the eye that controls the amount of light entering the eye by adjusting the pupil size.
3. Pupil – A small opening in the iris that allows light to enter the eye.
4. Lens – A convex lens that focuses light on the retina and helps in adjusting the focal length to see objects at different distances (accommodation).
5. Retina – A light-sensitive screen at the back of the eye that forms the image of objects. It contains photoreceptor cells:
i) Rods (detect dim light, night vision)
ii) Cones (detect colors, bright light vision)
6. Optic Nerve – Transmits the image signal from the retina to the brain for interpretation.
Power of Accommodation
The ability of the eye lens to adjust its focal length to see objects at different distances is called accommodation. The ciliary muscles control this adjustment.
Near Point: The closest distance at which an object can be seen clearly (usually 25 cm for a normal eye).
Far Point: The farthest distance at which an object can be seen clearly (infinity for a normal eye).
2. Defects of Vision and Their Correction
There are three common defects of vision:
(i) Myopia (Near-sightedness)
The person can see nearby objects clearly but cannot see distant objects.
Cause:
i) The eyeball is too long.
ii) The eye lens is too curved (focal length is too short).
Correction:
A concave lens is used to diverge the light rays before they enter the eye, so they focus on the retina.
(ii) Hypermetropia (Far-sightedness)
The person can see distant objects clearly but cannot see nearby objects.
Cause:
i) The eyeball is too short.
ii) The eye lens is too flat (focal length is too long).
Correction:
A convex lens is used to converge the light rays before they enter the eye, so they focus on the retina.
(iii) Presbyopia
A defect that occurs with aging where both near and far vision are affected.
Cause:
Weakening of ciliary muscles and loss of lens flexibility.
Correction:
Bifocal lenses (upper part concave for distance, lower part convex for reading).
3. Refraction of Light Through a Prism
A prism is a transparent glass object with a triangular shape.
When light passes through a prism, it bends (refracts) twice – once when entering and again when leaving.
The bending causes the light to spread out into different colors, a phenomenon called dispersion of light.
4. Dispersion of Light and the Spectrum
Dispersion of Light: The splitting of white light into its seven colors when it passes through a prism.
The seven colors are VIBGYOR:
Violet
Indigo
Blue
Green
Yellow
Orange
Red
Cause: Different colors of light have different wavelengths and bend by different amounts while passing through the prism.
Rainbow Formation: A natural example of dispersion caused by water droplets acting as tiny prisms in the atmosphere.
5. Atmospheric Refraction
Definition: The bending of light due to varying densities of air in the atmosphere.
Examples in Daily Life:
1. Twinkling of Stars:
Light from stars passes through layers of air with different densities.
Continuous bending of light makes the star appear to twinkle.
2. Apparent Position of the Sun:
The sun appears slightly above its actual position due to refraction by the Earth’s atmosphere.
The sun is visible before sunrise and after sunset due to atmospheric refraction.
6. Scallering of Light
Definition: The spreading of light in different directions due to particles in the atmosphere.
Tyndall Effect: The phenomenon of light scattering by small particles in a colloidal solution.
Examples in Daily Life:
1. Why is the sky blue?
Answer:
Blue light scatters more than red light because of its shorter wavelength.
Our eyes see the scattered blue light from the atmosphere.
2. Why does the sun appear red at sunrise and sunset?
Answer:
During sunrise and sunset, sunlight has to pass through a thicker layer of the atmosphere.
The shorter wavelengths (blue, violet) scatter out, and only red light reaches our eyes.
3. Why do clouds appear white?
Answer:
Water droplets and dust in clouds scatter all colors equally, making them appear white.
7. Natural Phenomena Related to Light
1. Rainbow Formation:
Sunlight gets dispersed by raindrops, acting as tiny prisms.
The light refracts, reflects internally, and refracts again to form a rainbow.
2. Red Sunset and Sunrise:
Due to the scattering of shorter wavelengths, red light dominates during sunrise and sunset.
3. Blue Color of the Sky:
Blue light scatters the most due to its shorter wavelength.
Chapter 11: The Human Eye and the Colourful World of the NCERT Class 10 Science textbook:
Q. What is meant by the power of accommodation of the eye?
Answer:
The power of accommodation of the eye refers to the ability of the eye lens to adjust its focal length to clearly focus objects at varying distances on the retina. This adjustment allows us to see both near and distant objects clearly.
Q. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What type of corrective lens should be used to restore proper vision?
Answer:
A person with myopia (nearsightedness) should use a concave (diverging) lens with a focal length of 1.2 meters to correct their vision. This lens helps in diverging light rays before they enter the eye, ensuring that the image is formed on the retina.
Q. What are the far point and near point of the human eye with normal vision?
Answer:
Near Point: The closest distance at which the eye can see an object clearly without strain, typically about 25 cm for a normal eye.
Far Point: The farthest distance at which the eye can see objects clearly, which is at infinity for a normal eye.
Q. 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?
Answer:
The student is likely suffering from myopia (nearsightedness), a condition where distant objects appear blurry. This can be corrected by using concave (diverging) lenses of appropriate power, which help focus distant images onto the retina.
Q. 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) far-sightedness
Answer: (b) accommodation
[Accommodation is the process by which the eye adjusts its focal length to focus on objects at various distances. ]
Q. The human eye forms the image of an object at its
(a) cornea
(b) iris
(c) pupil
(d) retina
Answer: (d) retina
[The retina is the light-sensitive layer at the back of the eye where images are formed and then transmitted to the brain via the optic nerve. ]
Q. 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
[The minimum distance at which a young adult with normal vision can see objects clearly without strain is approximately 25 cm.]
Q. 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
[The ciliary muscles adjust the curvature of the eye lens, thereby changing its focal length to focus on objects at different distances. ]
Q. 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 dioptres. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Answer:
(i) Distant Vision:
Power (P) = -5.5 D
Focal length (f) = 1/P = 1/(-5.5) = -0.181 m = -181 mm
So, a concave lens with a focal length of -181 mm is required.
(ii) Near Vision:
Power (P) = +1.5 D
Focal length (f) = 1/P = 1/1.5 = +0.667 m = +667 mm
So, a convex lens with a focal length of +667 mm is required.
Q. 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?
Answer:
For a myopic person, the corrective lens should form an image of distant objects (at infinity) at the person's far point (80 cm).
Using the lens formula:
1/f = 1/v - 1/u
Here, u = ∞ (object at infinity), v = -80 cm (image at the far point; negative because it's a virtual image)
1/f = 1/(-80 cm) - 1/∞ = -1/80 cm
f = -80 cm = -0.8 m
Power (P) = 1/f (in meters) = 1/(-0.8) = -1.25 D
Therefore, a concave lens with a power of -1.25 dioptres is required to correct the myopia.
Q. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer:
A normal eye cannot see objects placed closer than 25 cm clearly because, at such short distances, the ciliary muscles cannot contract sufficiently to increase the curvature of the lens. This limitation prevents the eye from focusing the image on the retina, resulting in a blurred vision.
Q. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer:
As the distance of an object from the eye increases, the eye lens becomes thinner, increasing its focal length. However, the image distance within the eye remains constant because the eye adjusts the lens's focal length to ensure that the image is always formed on the retina.
Q. Why do stars twinkle?
Answer:
Stars twinkle due to atmospheric refraction. As starlight enters the Earth's atmosphere, it passes through layers of varying densities, causing continuous refraction. This refraction leads to fluctuations in the apparent position and brightness of stars, making them appear to twinkle.
Q. Explain why the planets do not twinkle.
Answer:
Planets do not twinkle because they are much closer to Earth compared to stars and can be considered as extended sources of light. The light from different parts of a planet's disk undergoes atmospheric refraction differently, and these variations average out, minimizing the twinkling effect.
Q. Why does the Sun appear reddish early in the morning?
Answer:
During sunrise and sunset, the Sun's light has to pass through a greater thickness of the Earth's atmosphere. Shorter wavelengths (blue and violet) are scattered out, leaving the longer wavelengths (red and orange) to reach our eyes, making the Sun appear reddish.
Q. Why does the sky appear dark instead of blue to an astronaut?
Answer:
The sky appears dark to astronauts because, in space, there is no atmosphere to scatter sunlight. The blue color of the sky on Earth is due to the scattering of sunlight by the atmosphere. In the absence of an atmosphere, as in space, there is no scattering, and the sky appears dark.
Thank you 🙏.
See also
1) Magnetic fields effects of electric current
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