Anatomy of the Eye & Power of Accommodation
- Cornea: Transparent spherical membrane on front surface; refracts ~$80\%$ of incoming light.
- Iris & Pupil: Iris regulates pupil aperture to control light intensity entering the eye.
- Crystalline Lens: Flexible double convex lens whose curvature is adjusted by ciliary muscles.
- Retina: Light-sensitive inner screen containing rods (light intensity) and cones (color vision).
- Power of Accommodation: Ability of the eye lens to adjust its focal length to focus objects clearly:
- Near Point of Normal Eye: $25\text{ cm}$ (Least distance of distinct vision).
- Far Point of Normal Eye: Infinity ($\infty$).
Defects of Vision & Optical Corrections
| Defect | Symptoms & Underlying Causes | Correction Method |
|---|---|---|
| Myopia (Near-sightedness) | Can see near objects clearly; cannot focus distant objects. Caused by excessive lens curvature or elongation of eyeball. Image forms in front of retina. | Concave Lens of appropriate diverging power. |
| Hypermetropia (Far-sightedness) | Can see distant objects clearly; near objects blurry. Caused by focal length being too long or eyeball too short. Image forms behind retina. | Convex Lens of appropriate converging power. |
| Presbyopia (Old-age sight) | Aging ciliary muscles lose elasticity. Difficulty focusing near objects. | Bifocal Lenses (Upper concave for distance; lower convex for reading). |
Atmospheric Phenomena
- Dispersion of Light: Splitting of white light into its $7$ constituent spectral colors ($VIBGYOR$) through a glass prism because different wavelengths travel at different speeds in glass (Red deviates least, Violet deviates most).
- Twinkling of Stars: Light from distant point-like stars passes through turbulent atmospheric layers of varying refractive indices, continuously shifting the apparent star position.
- Advanced Sunrise & Delayed Sunset: Atmospheric refraction bends sunlight around Earth's horizon, causing the Sun to appear visible $2$ minutes before actual sunrise and $2$ minutes after sunset (Total extension $= 4\text{ mins}$).
- Scattering of Light & Rayleigh's Law: Scattering intensity is inversely proportional to the fourth power of wavelength: $I \propto \frac{1}{\lambda^4}$.
- Blue Sky: Blue light has short wavelength and scatters 16x more strongly than red.
- Red Sunset: Sun rays travel the longest distance through atmosphere; shorter wavelengths scatter away, leaving only longer red wavelengths to reach our eyes.
