👁️ Introduction

This chapter explains the structure and working of the human eye and various phenomena related to light such as refraction through a prism, dispersion, scattering, and atmospheric effects.

👀 1. Structure of the Human Eye

The human eye acts like a camera and helps us see the world around us.

🔹 Parts of the Eye:

  • Cornea: Transparent front part that refracts light.

  • Iris: Colored part controlling the size of the pupil.

  • Pupil: Opening through which light enters.

  • Lens: Convex lens that focuses light on the retina.

  • Retina: Light-sensitive layer where image forms.

  • Ciliary Muscles: Change the shape of the lens to focus.

  • Optic Nerve: Carries visual information to the brain.

🔹 Image Formation:

  • Light enters through the pupil.

  • Lens focuses light on retina.

  • Image is real and inverted.

  • Brain interprets the image as upright.

🔄 2. Accommodation of the Eye

The ability of the eye lens to adjust its focal length to focus on near and distant objects is called accommodation.

  • For distant objects: Lens becomes thin.

  • For nearby objects: Lens becomes thick.

🔹 Limits of Accommodation:

  • Near Point: 25 cm (minimum distance to see clearly)

  • Far Point: Infinity

👓 3. Common Defects of Vision

  1. Myopia (Short-sightedness):

    • Can see nearby objects clearly

    • Distant objects appear blurred

    • Caused by elongated eyeball or lens too curved

    • Corrected by concave lens

  2. Hypermetropia (Long-sightedness):

    • Can see distant objects clearly

    • Nearby objects appear blurred

    • Caused by short eyeball or lens too flat

    • Corrected by convex lens

  3. Presbyopia:

    • Occurs with aging

    • Eye loses flexibility to focus

    • Corrected by bifocal lenses

  4. Cataract:

    • Clouding of the eye lens

    • Treated by surgery and lens replacement

🔺 4. Refraction through Prism

A prism bends light due to refraction. The emergent ray bends away from the base.

  • Angle of deviation: Angle between incident and emergent ray

🌈 5. Dispersion of Light

Dispersion: Splitting of white light into its 7 component colors (VIBGYOR).

  • Discovered by Isaac Newton

  • Occurs due to different refractive indices for different colors

🌆 6. Atmospheric Refraction and Scattering

🔹 Atmospheric Refraction:

  • Refraction of light due to varying air density in atmosphere

  • Effects:

    • Star appears twinkling

    • Sun appears earlier at sunrise and later at sunset

🔹 Scattering of Light:

  • Redirection of light by particles in the atmosphere

  • Tyndall Effect: Scattering of light by colloidal particles

🔹 Applications:

  • Sky appears blue due to scattering of blue light

  • Sun appears reddish at sunrise and sunset due to longer path and scattering of shorter wavelengths

✅ Key Points to Remember

  • Eye can focus due to lens flexibility (accommodation)

  • Myopia and hypermetropia are corrected using lenses

  • Prism shows dispersion of light into spectrum

  • Atmospheric phenomena explained by refraction and scattering