Here are clear answers to your questions from the chapter
🌈 The Human Eye and the Colourful World — Class 10 NCERT, CBSE Board:
✅ Q1.
The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to —
🎯 (b) accommodation.
✅ Q2.
The human eye forms the image of an object at its —
🎯 (d) retina.
✅ Q3.
The least distance of distinct vision for a young adult with normal vision is about —
🎯 (c) 25 cm.
✅ Q4.
The change in focal length of an eye lens is caused by the action of the —
🎯 (c) ciliary muscles.
✅ Q5.
A person needs a lens of power –5.5 D for distant vision and +1.5 D for near vision. What are the focal lengths of these lenses?
- Formula: f = \frac{100}{P} (in cm)
- For distant vision: P = -5.5 ⇒ f = \frac{100}{-5.5} ≈ -18.18 cm = –0.1818 m
- For near vision: P = +1.5 ⇒ f = \frac{100}{1.5} ≈ 66.7 cm = 0.667 m
✅ Q6.
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 myopia, we use a concave lens.
- u = -\infty, v = -80 cm.
- \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{-80} - 0 = -\frac{1}{80} m.
- f ≈ -0.8 m ⇒ P = \frac{100}{-80} = -1.25 D.
🎯 Lens: Concave, power ≈ –1.25 D
✅ Q7.
Make a diagram to show how hypermetropia is corrected.
📐 A convex lens is used to converge light rays so they focus properly on the retina.
Calculation:
- Near point of hypermetropic eye = 1 m
- Near point of normal eye = 25 cm.
- u = –25 cm, v = –100 cm.
- \frac{1}{f} = \frac{1}{v} – \frac{1}{u} = \frac{1}{–100} – \frac{1}{–25} = –0.01 + 0.04 = 0.03 cm^{-1}
- f ≈ 33.3 cm ⇒ P ≈ +3 D
🎯 Lens: Convex, power ≈ +3 D
✅ Q8.
Why can’t a normal eye see objects closer than 25 cm?
- Because the ciliary muscles cannot make the eye lens sufficiently thick beyond a certain limit to focus very near objects.
✅ Q9.
What happens to the image distance in the eye when we increase the distance of an object?
- The image distance in the eye remains fixed (equal to the distance between lens and retina), and the focal length of the eye lens is adjusted by the ciliary muscles.
✅ Q10.
Why do stars twinkle?
- Due to the refraction of starlight by the atmosphere which is constantly changing (atmospheric turbulence), making the star’s position and brightness appear to fluctuate.
✅ Q11.
Why don’t planets twinkle?
- Planets appear larger (finite disc) compared to point-sized stars, so the variations in atmospheric refraction average out and don’t produce noticeable twinkling.
✅ Q12.
Why does the sky appear dark instead of blue to an astronaut?
- In space, there is no atmosphere to scatter sunlight, so the sky appears black instead of blue.
📖 If you’d also like, I can prepare:
✨ Ray diagrams (for hypermetropia & myopia)
✨ A colorful HTML page
✨ Or a printable PDF of all these answers neatly formatted.
Would you like that? Let me know!