Class 10 Science – The Human Eye

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Class 10 Science – The Human Eye and The Colourful World (Easy Notes)

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1. Structure of the Human Eye

  • The eye works like a camera.
  • Cornea: The transparent layer in front that bends most of the incoming light.
  • Iris: The coloured part (brown, blue, black). It controls the size of the pupil.
  • Pupil: The black hole in the centre; it controls how much light enters the eye.
  • Lens: Clear, flexible structure that focuses light onto the retina.
  • Ciliary muscles: Tiny muscles that make the lens thick or thin to adjust focus.
  • Retina: Screen at the back of the eye. It has two cells:
    • Rods (for dim light)
    • Cones (for colours).
  • Optic nerve: Carries the image from retina to the brain.

 

2. Power of Accommodation

  • The eye can focus on far and near objects by changing the lens shape.
  • If you look at a book close to your eyes → lens becomes thicker.
  • If you look at a mountain far away → lens becomes thinner.
  • Near point: Closest distance you can see clearly = about 25 cm.
  • Far point: Farthest distance you can see clearly = infinity.

 

3. Defects of Vision and their Correction

  • Sometimes the eye cannot focus properly. These are called defects of vision.

1.    Myopia (Nearsightedness)

o   You can see near things clearly, but far things look blurred.

o   Cause: Eyeball too long or lens too curved.

o   Correction: Concave lens glasses.

2.    Hypermetropia (Farsightedness)

o   You can see far things clearly, but near things look blurred.

o   Cause: Eyeball too short or lens too thin.

o   Correction: Convex lens glasses.

3.    Presbyopia

o   Happens in old age.

o   Eye muscles lose strength → difficulty in seeing near objects.

o   Correction: Bifocal lenses (top part concave for far vision, bottom part convex for near vision).

4.    Astigmatism

o   Eye cannot focus all directions equally → blurred vision.

o   Cause: Uneven curvature of lens or cornea.

o   Correction: Cylindrical lens glasses.

 

4. Refraction of Light through Atmosphere

  • Light bends (refracts) when it moves through air layers of different density.
  • Effects:
    • Stars twinkle: Their light bends due to moving air layers.
    • Planets don’t twinkle: They look bigger (many points of light).
    • Advance sunrise & delayed sunset: We see the sun earlier in the morning and longer in the evening because of atmospheric bending of sunlight.

 

5. Scattering of Light

  • Tiny particles in air scatter sunlight.
  • Scattering is stronger for short wavelengths (blue, violet) and weaker for long wavelengths (red).
  • Effects:
    • Blue sky: Blue light scatters the most, so the sky looks blue.
    • Red sunrise/sunset: When the sun is low, light travels a longer path, so only red/orange light reaches us.

 

6. Dispersion of Light

  • White light splits into 7 colours (VIBGYOR) when it passes through a prism.
  • This happens because each colour bends differently:
    • Violet bends the most.
    • Red bends the least.
  • That is why we get a spectrum.

 

7. Tyndall Effect

  • When light passes through a colloidal solution (tiny particles floating in air/water), it gets scattered.
  • Examples:
    • Sun rays visible through gaps in trees (dust particles scatter light).
    • Projectors using smoke or mist to show light beams.
    • Blue colour of smoke from motorbikes.

 

8. Rainbow Formation

  • Rainbows form due to refraction, dispersion, and total internal reflection inside water droplets in the air.
  • Steps:

1.    Sunlight enters a raindrop → bends and splits into colours.

2.    Light reflects inside the droplet.

3.    Light comes out, showing 7 colours.

  • Primary rainbow: Red outside, violet inside.
  • Secondary rainbow: Fainter, double reflection inside drops, colours reversed.

 

9. Practical Applications & Phenomena

  • Sky dark in space: No atmosphere, so no scattering → sky looks black.
  • Advance sunrise: We see the sun earlier due to atmospheric refraction.
  • Delayed sunset: We see the sun even after it sets below the horizon.

 

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NCERT CLASS 10 SCIENCE

Chapter: The Human Eye — Class 10 Science (NCERT).

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