Human eye and colourful world solutions Class 10 Physics NCERT

Human Eye and Colourful World - NCERT Class 10 Solutions

👁️ Human Eye and the Colourful World – NCERT Solutions

Let’s explore how our eyes work, what causes vision defects, and why the sky is blue! This chapter is filled with amazing facts and real-life science around us.

📘 In-text Questions – Page No. 164

1. What is meant by power of accommodation of the eye?

The power of accommodation of the eye is the ability of the eye lens to adjust its focal length to focus on objects at different distances.

This is achieved by the action of ciliary muscles, which change the curvature of the lens:

• To see nearby objects, the lens becomes thicker (more curved).
• To see distant objects, the lens becomes thinner (less curved).

Did you know? Our eyes can focus from 25 cm to infinity in just a fraction of a second!

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?

The person is suffering from myopia (near-sightedness). This means they can see nearby objects clearly but distant objects appear blurred.

To correct this defect, a concave lens (diverging lens) is used. It shifts the image of distant objects onto the retina by diverging the incoming rays before they enter the eye.

Pro Tip 🔍: Always get your eye power checked before using glasses. Wrong lenses can worsen the problem!

3. What is the far point of a normal human eye?

The far point of a normal human eye is the farthest distance at which an object can be seen clearly without strain.

For a healthy human eye, the far point is considered to be infinity (∞).

Fun Fact 🌍: We consider stars and planets as being at "infinity" because they are millions of kilometers away!

4. What is the near point of the human eye with normal vision?

The near point of a human eye with normal vision is the minimum distance at which an object can be seen clearly without strain.

This distance is about 25 cm for a normal eye.

📏 This is why books, phones, and reading material are ideally held at about 25 cm distance from the eyes.

🎯 NCERT Exercise – Multiple Choice Questions (MCQs)

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) far-sightedness

Answer: (b) accommodation

Hint 🧠: Ciliary muscles help change the shape of the lens to focus on near or far objects — that’s accommodation!

2. The human eye forms the image of an object at its:
(a) cornea
(b) iris
(c) pupil
(d) retina

Answer: (d) retina

The retina acts like a screen inside our eye where images are formed and then sent to the brain via the optic nerve.

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

📚 This is the comfortable distance at which most reading and writing is done.

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

💡 These muscles help focus the lens by altering its curvature.

📘 NCERT Exercise Questions – Page 170

5. 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?

Myopia (or near-sightedness) is a defect in which the person can see nearby objects clearly but distant objects appear blurry.

To correct this, we use a concave lens (also called a diverging lens), which helps diverge the incoming rays before they enter the eye.

🔎 Hint: Concave lenses spread light rays outward, allowing the image to focus on the retina.

💡 Pro Tip: The focal length of the lens must be chosen so the far point shifts from 1.2 m to infinity.

6. What is the far point of a person suffering from myopia?

The far point is the maximum distance at which a person can see clearly.

For a person with myopia, the far point is less than infinity. In the question above, the far point is 1.2 meters.

📌 In normal vision, the far point is at infinity. In myopia, the eye lens focuses light in front of the retina for distant objects.

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.

Hypermetropia (farsightedness) is a defect where a person cannot see nearby objects clearly, but can see distant objects well.

To correct this defect, we use a convex lens (also called a converging lens), which helps converge light rays onto the retina.

Given:
Near point of hypermetropic eye, `\( d = 1\ \text{m} = 100\ \text{cm} \)`
Near point of normal eye (u) = –25 cm (object is on the left of lens)
Image distance, `\( v = –100\ \text{cm} \)`

Using lens formula:
`\[ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{-100} - \frac{1}{-25} = -\frac{1}{100} + \frac{1}{25} = \frac{3}{100} \Rightarrow f = \frac{100}{3}\ \text{cm} ≈ 33.33\ \text{cm} \]`

Convert to meters: `\( f = 0.333\ \text{m} \)`

Power of lens:
`\[ P = \frac{100}{f(\text{cm})} = \frac{100}{33.33} ≈ +3\ \text{D} \]`

🔍 Answer: The required lens is a convex lens of power +3D.

Diagram: kindly refer hypermetropia correction ray diagram

🌟 NCERT Exercise Questions – Page 170 (Continued)

8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

The minimum distance at which the eye can focus clearly without strain is called the least distance of distinct vision, which is 25 cm for a normal eye.

If an object is placed closer than 25 cm, the ciliary muscles cannot contract further to increase the curvature of the eye lens. Hence, the image cannot be formed clearly on the retina, and the object appears blurred.

👁️ Key Concept: Closer objects need more converging power to focus the image on the retina.

9. What happens to the image distance in the eye when we increase the distance of an object from the eye?

When we increase the distance of the object from the eye:

• The image distance does not change.
• The image is always formed on the retina at a fixed distance inside the eye.
• The focal length of the lens changes by adjusting the curvature of the eye lens to focus the image properly.

🔍 Fun Fact: The human eye automatically adjusts its focal length using ciliary muscles – a process called accommodation!

10. Why do stars twinkle?

Stars twinkle due to a phenomenon called atmospheric refraction.

As starlight enters Earth's atmosphere, it passes through layers of air with different densities. These varying layers refract (bend) the light path irregularly.

This causes the star’s position and brightness to change slightly and rapidly, making it appear as if the star is twinkling.

🌠 Important: Planets do not twinkle because they appear as extended sources of light and their light averages out the twinkling.

11. Explain why the planets do not twinkle.

Planets are much closer to the Earth than stars and appear as extended sources of light.

The light from a planet comes from multiple points, so the effects of atmospheric refraction from different paths cancel each other out.

Hence, their overall brightness remains steady, and they do not twinkle like stars.

🪐 Key Idea: Twinkling is noticeable only when light comes from a point source, like stars.

12. Why does the sun appear reddish early in the morning?

At sunrise, sunlight has to travel a longer distance through the Earth's atmosphere to reach our eyes.

During this longer path, shorter wavelengths (blue and violet) are scattered out by the particles in the atmosphere.

Only the longer wavelengths (red and orange) reach us, making the sun appear reddish.

🌅 Fun Fact: The same reason causes sunsets to appear red or orange!

🌟 NCERT Exercise Questions – Page 170 (Final Part)

13. Why does the sky appear blue?

The sky appears blue due to a phenomenon called scattering of light.

The air molecules and dust particles in Earth’s atmosphere scatter sunlight. Shorter wavelengths (blue and violet) are scattered more than the longer wavelengths (like red).

Although violet is scattered even more than blue, our eyes are more sensitive to blue light, and some violet is absorbed by the upper atmosphere. Hence, we see the sky as blue.

☁️ Key Concept: Scattering is inversely proportional to the fourth power of the wavelength (Rayleigh scattering).

14. The clear sky appears blue but the clouds appear white. Why?

Clouds consist of tiny water droplets or ice crystals that are much larger than the wavelength of visible light.

These particles scatter all wavelengths almost equally, so the scattered light remains white.

Hence, clouds appear white, while the clear sky appears blue due to selective scattering.

15. Why does the sky appear dark instead of blue to an astronaut?

In space, there is no atmosphere to scatter sunlight. Without scattering, the sky appears black or dark to astronauts even in daytime.

This is why astronauts in space or on the moon observe a pitch-black sky instead of a blue one.

🚀 Science Insight: Scattering requires particles; vacuum in space has none!

16. Explain the formation of a rainbow.

A rainbow is formed due to the combination of refraction, dispersion, and total internal reflection of sunlight in water droplets present in the atmosphere.

1. Sunlight enters a water droplet and is refracted and dispersed into its component colors.
2. These rays are then reflected internally from the back of the droplet.
3. Finally, they refract again while coming out of the droplet, reaching the observer's eye.

🌈 Fun Fact: Each observer sees their own personal rainbow!

17. What is meant by the dispersion of white light? Describe the effect of dispersion of white light by a glass prism.

Dispersion of white light refers to the splitting of white light into its seven constituent colors (VIBGYOR) when it passes through a prism.

This occurs because each color has a different wavelength and bends (refracts) by a different amount. Violet bends the most and red the least.

In a glass prism, dispersion shows that white light is a mixture of multiple colors.

🎯 Final Words

This chapter beautifully connects human vision with natural phenomena like rainbows and blue skies. Understanding these concepts builds a strong foundation for optics and daily science wonders.

💡 Pro Tip for Exams: Use ray diagrams, keywords like "refraction", "dispersion", and "accommodation", and link answers to real-life examples wherever possible!