Human Eye and Its Structure

  • The human eye is a sensitive organ that helps us see the world.
  • It is like a camera, and its lens system forms an image on the retina, which is light-sensitive.
  • Light enters the eye through the cornea, a transparent bulge.
  • The cornea does most of the refraction of light, while the eye lens helps focus on objects at different distances.
  • Behind the cornea, there is a structure called the iris, a dark muscular diaphragm that controls the size of the pupil (the opening through which light enters).
  • The image formed by the eye lens is inverted (upside down) on the retina.
  • The retina has light-sensitive cells that send signals to the brain through the optic nerve, allowing us to perceive images.

2. Power of Accommodation

  • The eye lens can change its shape to focus on objects at different distances.
  • The ciliary muscles control the lens’s curvature:
    • When relaxed, the lens is thin, and we can see distant objects.
    • When contracted, the lens becomes thicker, allowing us to see nearby objects clearly.
  • The minimum distance at which objects can be seen clearly is called the “least distance of distinct vision” (near point), which is 25 cm for a normal eye.
  • The farthest point a normal eye can see clearly is infinity.
  • In older people, the eye lens may become cloudy, causing cataracts, which can be treated through surgery.

3. Defects of Vision
There are three main vision defects:

  • Myopia (Near-sightedness):
    • People with myopia can see nearby objects clearly but not distant ones.
    • The image of distant objects is formed in front of the retina due to excessive lens curvature or an elongated eyeball.
    • It can be corrected with a concave lens (diverges light).
  • Hypermetropia (Far-sightedness):
    • People with hypermetropia can see distant objects clearly but struggle with nearby objects.
    • The image of nearby objects is formed behind the retina.
    • It is corrected with a convex lens (converges light).
  • Presbyopia:
    • This occurs with aging when the eye loses its ability to focus on nearby objects.
    • It happens due to the weakening of the ciliary muscles and decreased flexibility of the eye lens.
    • It is often treated with bi-focal lenses (concave for distance and convex for near).

4. Other Points on Vision and Eye Care

  • Eye Donation: Donating eyes can help restore vision for people who are blind, especially children.
  • Eye donation is possible for people of all ages, including those who wear glasses or have had cataract surgery.
  • Eyes should be donated within 4-6 hours of death.

5. Refraction of Light Through a Prism

  • A triangular prism has three sides, and light passing through it bends at two surfaces.
  • The bending of light at each surface creates an angle of deviation.
  • In an experiment with sunlight passing through a prism, a band of colors (spectrum) is seen due to the dispersion of light.
  • The angle at which light enters and exits the prism is called the angle of incidence and angle of refraction.

Additional Knowledge for Competitive Exams:

  • Power of Accommodation: This is the ability of the eye to adjust the focal length and focus on objects at different distances. It helps in understanding the process of vision correction.
  • Refractive Errors: Myopia, hypermetropia, and presbyopia are common refractive defects. Understanding these defects and their corrections is important for both practical applications (like glasses) and theoretical questions in exams.
  • Prism and Dispersion: When white light passes through a prism, it splits into its constituent colors. This phenomenon is called dispersion. It is important to understand how light refracts and how different materials can change the direction of light.

Dispersion of White Light by a Glass Prism

  • When white light passes through a glass prism, it gets split into a band of colors, known as the spectrum.
  • These colors appear as Violet, Indigo, Blue, Green, Yellow, Orange, and Red (remembered as VIBGYOR).
  • Dispersion happens because different colors bend by different amounts when passing through the prism. Red light bends the least, while violet bends the most.
  • Isaac Newton first used a glass prism to split sunlight into these colors. By using a second prism, he showed that white light is made up of these seven colors.
  • A rainbow is a natural spectrum formed when sunlight passes through water droplets in the atmosphere. These droplets act like tiny prisms, causing the sunlight to refract and disperse, forming the rainbow.

Atmospheric Refraction

  • Atmospheric refraction happens when light travels through air of varying temperature and density, bending the light path. This causes effects like:
    • Twinkling of stars: Stars appear to flicker because their light refracts as it passes through Earth’s atmosphere.
    • Why planets don’t twinkle: Planets are closer and appear as extended sources of light, so the light variations cancel out.
    • Advance sunrise and delayed sunset: The Sun appears about 2 minutes earlier than its actual sunrise time and 2 minutes later than sunset due to atmospheric refraction.

Scattering of Light

  • Tyndall Effect: The scattering of light by small particles (like dust or water droplets) makes the light beam visible in the air, as seen in a smoky room or a forest with mist.
  • Why the Sky is Blue: Air molecules scatter short-wavelength (blue) light more than long-wavelength (red) light, making the sky appear blue. At high altitudes, the sky looks dark because scattering is weaker.
  • Red Signal Lights: Red light scatters less than other colors, making it visible from a distance even in fog or smoke.

Key Takeaways

  • Accommodation of the Eye: The eye can focus on both near and distant objects by adjusting its focal length.
  • Vision Defects: Myopia (short-sightedness) and hypermetropia (far-sightedness) are corrected with concave or convex lenses, respectively.
  • Dispersion: The splitting of white light into its component colors is called dispersion.
  • Scattering: Light scattering causes the blue color of the sky and helps explain the appearance of different colored lights in certain conditions.

THESE ALL ARE THE NOTES OF CHAPTER 10. AND AFTER SOME TIME YOU GET IMPORTANT QUESTIONS AND DIAGRAMS HERE. *#THANKS FOR VISITING, VISIT AGAIN#* 😊