Structure of eye, defects of vision, atmospheric refraction, dispersion, and scattering of light
Cornea: Transparent outermost layer; most refraction happens here (~66%)
Iris: Coloured muscular ring; controls size of pupil
Pupil: Opening in iris; dilates in dim light, contracts in bright light
Eye lens: Convex, flexible biconvex lens; fine-tunes focus by changing shape (accommodation)
Ciliary muscles: Muscles attached to eye lens; contract/relax to change focal length
Retina: Light-sensitive layer at back; contains two types of cells:
ā¢ Rods ā detect dim light (black and white vision)
ā¢ Cones ā detect colour (require bright light); concentrated at fovea (point of sharpest vision)
Optic nerve: Carries visual signals from retina to brain
Blind spot: Where optic nerve exits ā no rod/cone cells here, so no vision at this point
The ability of the eye lens to change its focal length (by adjusting curvature) to focus on objects at varying distances is called accommodation.
Near object ā ciliary muscles contract ā lens becomes thicker (more curved) ā shorter focal length
Far object ā ciliary muscles relax ā lens becomes thinner (less curved) ā longer focal length
Near point: Minimum distance for clear vision = 25 cm (for normal eye)
Far point: Maximum distance for clear vision = infinity (for normal eye)
| Defect | Problem | Cause | Correction |
|---|---|---|---|
| Myopia (Near-sightedness) | Cannot see distant objects clearly | Image forms in front of retina (eyeball too long or lens too convex) | Concave lens (diverging) |
| Hypermetropia (Far-sightedness) | Cannot see near objects clearly | Image forms behind retina (eyeball too short or lens too flat) | Convex lens (converging) |
| Presbyopia | Cannot see both near and far | Age-related: ciliary muscles weaken, lens flexibility reduces | Bifocal lenses |
| Astigmatism | Blurred vision (both near and far) | Irregular curvature of cornea | Cylindrical lenses |
The atmosphere has different layers with different densities ā light refracts as it passes through.
Stars are very distant ā appear as point sources of light. As starlight passes through atmosphere with continuously changing density, it refracts along different paths ā star appears to twinkle (scintillation).
Planets don't twinkle ā they are closer ā appear as discs, and average fluctuations cancel out.
When sun is just below horizon, its light bends (refracts) toward Earth due to atmospheric refraction ā we see the sun before it actually rises and after it actually sets.
This adds approximately 2 minutes to the apparent day length!
When white light passes through a glass prism, it splits into its component colours: this is called dispersion.
The band of colours formed is called a spectrum: VIBGYOR
Violet, Indigo, Blue, Green, Yellow, Orange, Red
Violet is deviated most (shortest wavelength) | Red is deviated least (longest wavelength)
Refractive index: n_violet > n_red
A rainbow is formed due to dispersion, refraction, and total internal reflection of sunlight by raindrops.
Sunlight ā enters raindrop ā refracts ā reflects internally ā refracts again as it exits ā dispersed into colours.
Red is seen at top (less deviation), Violet at bottom (more deviation).
Rainbow is always seen with sun at your back!
When light passes through a colloidal solution (fine particles suspended in medium), it scatters. This is called the Tyndall Effect.
Examples: Sunlight through forest canopy (visible beam), car headlights in fog, light through smoky room.
Red colour has the longest wavelength ā scattered least by atmosphere ā travels farthest ā can be seen from greatest distance. That's why traffic signals, fire engines, danger signs, and railway signals are RED ā maximum visibility even in fog!