The Human Eye and the Colourful World · Class 10 Science: 88 flashcards
All 88 flashcards from this kit, free and with no sign-up.
1. Name the part of the eye where MOST refraction of light takes place.
Show the answer
The cornea — its curved transparent surface bends most of the incoming light.
2. What is the function of the iris?
Show the answer
A dark ring of muscle that opens and closes the pupil, adjusting how much light enters the eye.
3. What is the pupil?
Show the answer
The small round opening in the centre of the iris through which light enters the eye. It looks black because light entering it is absorbed inside.
4. What happens to the pupil in dim light?
Show the answer
It expands (becomes larger) so that more light can enter the eye.
5. Where is the image formed in the eye, and what is its nature?
Show the answer
On the retina — the image is real and inverted.
6. Which cells of the retina detect colour?
Show the answer
Cone cells — they work in bright light and give colour vision. (Rods work in dim light.)
7. What carries visual signals from the retina to the brain?
Show the answer
The optic nerve.
8. Define the power of accommodation.
Show the answer
The eye lens's ability to alter its own focal length, so that objects at different distances can be focused on the retina.
9. Which muscles change the shape of the eye lens?
Show the answer
The ciliary muscles — they contract to make the lens thicker and relax to let it become thinner.
10. What happens to the eye lens when you look at a DISTANT object?
Show the answer
Ciliary muscles relax → the lens becomes thin → its focal length increases → the distant object focuses on the retina.
11. What happens to the eye lens when you read a book?
Show the answer
Ciliary muscles contract → the lens becomes thick → its focal length decreases → the nearby page focuses on the retina.
12. State the near point (least distance of distinct vision) of a normal young eye.
Show the answer
25 cm.
13. State the far point of a normal eye.
Show the answer
Infinity — a normal eye can clearly see objects at any large distance.
14. What is a cataract?
Show the answer
A condition, usually in old age, where the crystalline lens becomes cloudy or milky, making vision blurred or lost. It is cured by surgically replacing the lens.
15. Can spectacles cure a cataract?
Show the answer
No — light is blocked inside the cloudy lens itself, so only surgical replacement of the lens restores vision.
16. Why is it difficult to read a book held 15 cm from the eye?
Show the answer
15 cm is closer than the near point (25 cm). The lens cannot shorten its focal length enough, so a clear image cannot form on the retina.
17. Give two advantages of having two eyes.
Show the answer
(1) A wider field of view (about 180° instead of about 150°). (2) Three-dimensional/depth vision, because the brain merges two slightly different views.
18. How does the eye differ from a camera in the way it focuses?
Show the answer
A camera moves its lens (changes image distance); the eye keeps the lens-retina distance fixed and changes the FOCAL LENGTH of its flexible lens instead.
19. What fills the space between the eye lens and the retina?
Show the answer
The vitreous humour — a transparent jelly-like substance.
20. What is the aqueous humour?
Show the answer
The thin watery liquid filling the space between the cornea and the eye lens.
21. Why does the pupil appear black?
Show the answer
Nearly all the light entering the pupil is absorbed inside the eye; since almost none comes back out, the opening looks black.
22. Which retinal cells help you see in dim light?
Show the answer
Rod cells.
23. What is the approximate diameter of the human eyeball?
Show the answer
About 2.3 cm.
24. What is the least distance of distinct vision also called?
Show the answer
The near point of the eye — 25 cm for a normal young adult.
25. What is myopia? What is its other name?
Show the answer
Short-sightedness — the defect in which nearby objects are seen clearly but distant objects appear blurred.
26. Where does the image form in a myopic eye viewing a distant object?
Show the answer
In front of the retina — the eye converges the light too strongly.
27. State the two causes of myopia.
Show the answer
(1) The eyeball is too long. (2) The eye lens is too curved, making its focal length too short.
28. Which lens corrects myopia?
Show the answer
A concave (diverging) lens of suitable negative power.
29. What happens to the far point in myopia?
Show the answer
It shifts from infinity to a finite nearby distance — the person cannot see clearly beyond it.
30. What is hypermetropia? What is its other name?
Show the answer
Far-sightedness (long-sightedness) — the defect in which distant objects are clear but nearby objects appear blurred.
31. Where would the image form in a hypermetropic eye viewing a near object?
Show the answer
Behind the retina — the eye converges the light too weakly.
32. State the two causes of hypermetropia.
Show the answer
(1) The eyeball is too short. (2) The eye lens is too flat, making its focal length too long.
33. Which lens corrects hypermetropia?
Show the answer
A convex (converging) lens of suitable positive power.
34. What happens to the near point in hypermetropia?
Show the answer
It recedes beyond 25 cm — the person must hold reading material farther away to see it clearly.
35. What is presbyopia and what causes it?
Show the answer
The age-related loss of the eye's power of accommodation, caused by gradual weakening of the ciliary muscles and loss of flexibility of the lens — the near point recedes.
36. What are bifocal lenses and who needs them?
Show the answer
Spectacle lenses with two parts — upper concave for distant vision, lower convex for reading. Needed by people who have both myopia and presbyopia.
37. Write the formula for the power of a lens with its unit.
Show the answer
P = 1/f, where f is in metres; the unit is the dioptre (D). 1 D = power of a lens of focal length 1 m.
38. A myopic person's far point is 2 m. What lens does he need?
Show the answer
f = −2 m → P = 1/(−2) = −0.5 D. A concave lens of power −0.5 D.
39. A lens of power +2 D — what type is it, and what defect does it likely correct?
Show the answer
Convex (positive power), focal length +0.5 m. It corrects hypermetropia or presbyopia.
40. What does the concave correcting lens do for a myopic eye, in one line?
Show the answer
It forms a virtual image of an infinitely distant object at the eye's far point — from where the myopic eye can focus it onto the retina.
41. What does the convex correcting lens do for a hypermetropic reader, in one line?
Show the answer
It forms a virtual image of an object at 25 cm at the eye's receded near point — from where the eye can focus it.
42. Distinguish hypermetropia from presbyopia in one line.
Show the answer
Hypermetropia comes from a short eyeball or flat lens (any age); presbyopia comes from ageing — weak ciliary muscles and a stiff lens.
43. A prescription reads −1.25 D. Find the focal length and defect.
Show the answer
f = 1/(−1.25) = −0.8 m = −80 cm; a concave lens correcting myopia (far point at 80 cm).
44. Can myopia and hypermetropia occur in the same person at once in one eye?
Show the answer
No — one eye either over-converges (myopia) or under-converges (hypermetropia) for a given state. But myopia + presbyopia together is common in older people, needing bifocals.
45. Why is the myopia correcting-lens formula simply f = −(far point)?
Show the answer
The lens must take parallel rays from infinity and make them appear to come from the far point — i.e. form a virtual image at distance d_far on the same side. A concave lens does this when |f| = d_far.
46. What is 1 dioptre?
Show the answer
The power of a lens of focal length 1 metre.
47. What is the angle of the prism?
Show the answer
The angle between the two plane refracting surfaces of the prism — labelled A.
48. Define the angle of deviation.
Show the answer
The angle between the emergent ray and the direction of the original incident ray — the total turning the prism gives to the light.
49. In which direction does a prism bend light at each surface?
Show the answer
Towards the base of the prism — at both refracting surfaces.
50. Why does a glass slab produce no net deviation while a prism does?
Show the answer
A slab's faces are parallel, so the second refraction exactly cancels the first. A prism's faces are inclined, so the two bendings add up.
51. What is dispersion of light?
Show the answer
The splitting of white light into its component colours (VIBGYOR) when it passes through a prism.
52. What is a spectrum?
Show the answer
The band of seven colours obtained when white light is dispersed by a prism.
53. Write the seven colours of the spectrum in order of decreasing deviation.
Show the answer
Violet, Indigo, Blue, Green, Yellow, Orange, Red — VIBGYOR.
54. Which colour of light deviates the most in a prism, and why?
Show the answer
Violet — it has the shortest wavelength and travels slowest in glass, so its refractive index is the highest.
55. Which colour deviates the least?
Show the answer
Red — it travels fastest in glass among the visible colours, so it bends the least.
56. What is the basic cause of dispersion?
Show the answer
Different colours of light travel at different speeds in glass, so each colour has a different refractive index and bends by a different amount.
57. How did Newton show that a prism does not create colours?
Show the answer
He placed a second identical prism upside-down in the path of the spectrum; it recombined the colours into white light — so the colours must already exist in white light.
58. What conclusion follows from Newton's two-prism experiment?
Show the answer
White light is a mixture of seven colours; a prism only separates (or re-mixes) them, it does not create them.
59. List the three steps inside a raindrop that create a rainbow.
Show the answer
(1) Refraction (with dispersion) as sunlight enters the drop, (2) internal reflection at the back surface, (3) refraction again as the light exits.
60. Where must the Sun be for you to see a rainbow?
Show the answer
Behind you — the rainbow always forms in the part of the sky opposite the Sun.
61. Name four factors on which the angle of deviation depends.
Show the answer
The angle of incidence, the angle of the prism, the refractive index of the prism's material, and the colour (wavelength) of the light.
62. In the spectrum on a screen (prism base down), which colour is at the top?
Show the answer
Red — because it deviates the least. Violet, deviating most towards the base, lands at the bottom.
63. Why does a diamond show flashes of colour?
Show the answer
Its very high refractive index produces very strong dispersion, separating the colours widely — seen as 'fire'.
64. Do all colours travel at the same speed in vacuum?
Show the answer
Yes — 3 × 10⁸ m/s for all. Speeds differ only inside a medium like glass or water, which is why dispersion needs a medium.
65. What is a rainbow, in one line?
Show the answer
A natural spectrum formed when sunlight is dispersed by water droplets in the atmosphere after rain.
66. Why is the rainbow curved?
Show the answer
Only drops lying on a particular cone of directions (about 40–42° from the anti-Sun point) send colour to your eye — that set of directions forms a circular arc.
67. What is atmospheric refraction?
Show the answer
The gradual bending of light as it passes through the atmosphere's layers of continuously changing density and refractive index.
68. Why does the atmosphere refract light at all?
Show the answer
Air density decreases with height, so the refractive index changes continuously — light bends gradually instead of travelling straight.
69. Does a star appear at its true position? Explain.
Show the answer
No — atmospheric refraction bends the starlight, making the star appear slightly HIGHER than its actual position.
70. Why do stars twinkle?
Show the answer
The atmosphere's shifting layers refract starlight differently from moment to moment, so the star's apparent position and brightness keep fluctuating.
71. Why do planets NOT twinkle?
Show the answer
Planets are closer and act as extended sources — collections of many point sources whose individual flickers average out, keeping the total light steady.
72. What is meant by advance sunrise?
Show the answer
Due to atmospheric refraction we see the Sun about 2 minutes BEFORE it actually crosses the horizon at sunrise.
73. What is meant by delayed sunset?
Show the answer
Refraction keeps the Sun visible for about 2 minutes AFTER it has actually gone below the horizon.
74. By how much does atmospheric refraction lengthen the apparent day?
Show the answer
By about 4 minutes — roughly 2 at sunrise plus 2 at sunset.
75. Why does the Sun look flattened (oval) at sunrise and sunset?
Show the answer
Light from the Sun's lower edge passes through denser air and is refracted more than light from its upper edge, squashing the visible disc vertically.
76. What is scattering of light?
Show the answer
The phenomenon in which particles in the path of light absorb it and re-emit it in all directions.
77. State the Tyndall effect.
Show the answer
The scattering of light by colloidal particles (smoke, mist, dust) which makes the path of a light beam visible.
78. Give two everyday examples of the Tyndall effect.
Show the answer
(1) Sunbeams visible through mist under trees or through a window into a dusty room. (2) A torch/projector/headlight beam visible in fog or smoke.
79. State the relation between wavelength and amount of scattering.
Show the answer
The smaller the wavelength, the greater the scattering — blue light scatters much more than red.
80. Why is the sky blue?
Show the answer
Air molecules scatter the short-wavelength blue part of sunlight strongly in all directions; this scattered blue light reaches our eyes from every part of the sky.
81. Why isn't the sky violet, though violet scatters even more than blue?
Show the answer
Sunlight contains less violet than blue, and our eyes are much less sensitive to violet — so the sky appears blue.
82. Why does the Sun appear red at sunrise and sunset?
Show the answer
At the horizon its light crosses a much longer column of air; most of the blue light is scattered out of the beam on the way, and the surviving light is rich in red.
83. Why are 'danger' signal lights red?
Show the answer
Red has the longest visible wavelength and is scattered least by fog and smoke — so a red light remains visible from the greatest distance.
84. Why does the sky appear dark/black to an astronaut?
Show the answer
In space there is no atmosphere to scatter sunlight, so no scattered light reaches the eye from above — the sky stays black.
85. Why do clouds appear white?
Show the answer
Cloud droplets are much larger than the wavelength of light and scatter all colours almost equally — the mixture looks white.
86. Where is a star's twinkling strongest — overhead or near the horizon?
Show the answer
Near the horizon — the light there crosses the longest, most turbulent stretch of atmosphere.
87. At the moment you SEE the sun set, where is the actual Sun?
Show the answer
Already below the horizon — refraction bends its light over the edge, so you watch an image about 2 minutes after the true sunset.
88. Would stars twinkle if seen from the Moon?
Show the answer
No — the Moon has no atmosphere, so there is no refraction to disturb the starlight; stars shine steadily.
Your notes can be a kit like these
Every kit here was made from real study material. Point Veda at a chapter, a PDF, a photo or a link and get notes, flashcards, practice MCQs, games and a scored test — free, in 42 languages.
- Chapter notes
- Flashcards
- Practice MCQs
- Scored tests
- Exam prep
Get Veda free See what students get
Free to start · No ads · Works in 42 languages