Notes - Human Eye - Class 10

Human Eye & Colourful World

Overview: Eye Structure, Vision Defects, Prism, Dispersion, Atmospheric Refraction, Scattering.

1. The Human Eye

Cornea: Transparent front part. Refracts most light.
Iris: Dark muscular diaphragm. Controls pupil size.
Pupil: Adjusts light entering eye.
Lens: Crystalline convex lens. Focuses image on retina.
Retina: Light sensitive screen. Rods/Cones. Signals to brain via Optic Nerve.
Power of Accommodation: Ability of lens to adjust focal length. Near Point = 25 cm. Far Point = Infinity.

2. Defects of Vision

Myopia: Near-sightedness. Correction: Concave Lens.
Hypermetropia: Far-sightedness. Correction: Convex Lens.
Presbyopia: Old age sight. Weak ciliary muscles. Correction: Bifocal lens.

3. Natural Phenomena

Dispersion: Splitting of white light into 7 colors (VIBGYOR). E.g., Rainbow.
Atmospheric Refraction: Twinkling of Stars, Advance sunrise/Delayed sunset.
Scattering (Tyndall Effect): Blue sky (Blue scattered most), Reddish sun (Red reaches us).

NCERT In-Text Questions (Solved)

Set 1

Q1. What is meant by power of accommodation of the eye?

The ability of the eye lens to adjust its focal length (by changing its curvature with the help of ciliary muscles) to see both near and distant objects clearly is called power of accommodation.

Q2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. Which corrective lens?

The person is suffering from Myopia (Near-sightedness). To correct this, a **Concave lens** of suitable power is used to restore proper vision.

Q3. What is the far point and near point of the human eye with normal vision?

Far Point: Infinity.
Near Point: 25 cm from the eye.

Q4. A student has difficulty reading the blackboard while sitting in the last row. Defect?

The student is unable to see distant objects clearly, so he is suffering from Myopia (or Near-sightedness).
Correction: By using a concave lens of suitable focal length.

Exercise Solutions - Human Eye - Class 10

NCERT Exercise Questions

Complete solutions for Chapter 10 exercises.

Q1. The human eye can focus 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. Person needs lens of power -5.5 D for distant vision and +1.5 D for near vision. Focal length required for (i) distant (ii) near.

(i) Distant Vision: P = -5.5 D.
f = 1/P = 1/(-5.5) = -0.18 m = -18.18 cm.
(ii) Near Vision: P = +1.5 D.
f = 1/P = 1/(1.5) = +0.67 m = +66.67 cm.

Q6. Far point of myopic person is 80 cm. Nature and power of lens required?

For myopic eye, object at infinity (u = -infinity) must form image at far point (v = -80 cm).
1/f = 1/v - 1/u = 1/(-80) - 1/(-infinity) = -1/80 cm.
f = -80 cm = -0.8 m.
P = 1/f = 1/(-0.8) = -1.25 D.
Nature: Concave Lens.

Q7. Make a diagram to show how hypermetropia is corrected. Near point is 1m. Power of lens?

Object at normal near point (u = -25 cm) must form virtual image at person's near point (v = -100 cm).
1/f = 1/v - 1/u = 1/(-100) - 1/(-25) = -1/100 + 4/100 = 3/100.
f = 100/3 cm = +33.3 cm = +0.33 m.
P = 1/f = 1/0.33 = +3.0 D.
Nature: Convex Lens.

Q8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Because the ciliary muscles cannot contract beyond a certain limit. They cannot decrease the focal length of the eye lens below a certain minimum limit corresponding to the distance of 25 cm.

Q9. What happens to the image distance in the eye when we increase the distance of an object from the eye?

The image distance in the eye is fixed (distance between lens and retina). When object distance increases, the eye lens adjusts its focal length (becomes thinner) to keep the image formed exactly on the retina.

Q10. Why do stars twinkle?

Due to Atmospheric Refraction. The atmosphere has layers of varying densities and refractive indices which keep changing. Light from a star (point source) bends continuously. This fluctuating apparent position and varying intensity of light entering the eye causes twinkling.

Q11. Explain why the planets do not twinkle.

Planets are much close to Earth and are seen as extended sources (collection of large number of point sources). The total variation in the amount of light entering our eye from all the individual point sources averages out to zero, thereby nullifying the twinkling effect.

Q12. Why does the Sun appear reddish early in the morning?

During sunrise, sunlight travels a longer distance through the thick layers of atmosphere. Shorter wavelengths (blue/violet) scatter away by particles. Only longer wavelengths (Red) reach our eyes (Scattering of Light).

Q13. Why does the sky appear dark instead of blue to an astronaut?

At high altitudes or in space, there is no atmosphere and thus no scattering of light. Since no scattered light reaches the eyes of the astronaut, the sky appears dark.

Formulas & Facts - Human Eye - Class 10

Key Concepts & Facts

Defects of vision and optical phenomena.

Important Concepts

Near Point of Eye 25 cm

Far Point of Eye Infinity

Myopia Correction Concave Lens (-ve power)

Hypermetropia Correction Convex Lens (+ve power)

Dispersion Splitting of White Light

Scattering Tyndall Effect

50 Important Facts

1. Human eye is like a camera.

2. Cornea is the transparent front part.

3. Iris controls the size of pupil.

4. Pupil regulates amount of light entering eye.

5. Eye lens is a double convex lens.

6. Retina acts as the screen.

7. Rods (Intensity) and Cones (Color) on retina.

8. Blind spot has no receptor cells.

9. Ciliary muscles adjust lens curvature.

10. Power of accommodation: Adjusting focal length.

11. Least distance of distinct vision is 25 cm.

12. Far point of normal eye is infinity.

13. Myopia: Near-sightedness.

14. Myopia: Elongation of eye ball.

15. Myopia corrected by Concave lens.

16. Hypermetropia: Far-sightedness.

17. Hypermetropia: Eye ball too short.

18. Hypermetropia corrected by Convex lens.

19. Presbyopia: Old age sight defect.

20. Bifocal lens used for Presbyopia.

21. Cataract: Cloudy eye lens. Surgery needed.

22. Prism has two triangular bases.

23. Angle of prism is angle between lateral faces.

24. Dispersion: Splitting of white light into 7 colors.

25. VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red.

26. Red light bends the least (Long wavelength).

27. Violet light bends the most (Short wavelength).

28. Isaac Newton discovered spectrum.

29. Recombination: Inverted prism joins colors to white.

30. Rainbow is formed opposite to Sun.

31. Rainbow involves dispersion, refraction, internal reflection.

32. Atmospheric refraction: Bending of light by air layers.

33. Twinkling of stars is due to atmospheric refraction.

34. Planets don't twinkle (Extended sources).

35. Advanced sunrise (2 mins) due to refraction.

36. Delayed sunset (2 mins) due to refraction.

37. Tyndall effect: Scattering of light by colloidal particles.

38. Path of light visible in dusty room is Tyndall effect.

39. Scattering depends on wavelength.

40. Blue light scattered more (Short wavelength).

41. Sky appears blue due to scattering.

42. Space looks black (No atmosphere).

43. Clouds look white (Scatter all wavelengths equally).

44. Danger signals are Red (Least scattered).

45. Sun looks reddish at sunrise/set due to scattering.

46. Short wavelengths scattered away at horizon.

47. Persistence of vision: 1/16th of a second.

48. Eye donation is valuable.

49. Both eyes give stereoscopic vision (Depth).

50. Owl has more rods for night vision.

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