The Human Eye and the Colourful World — Class 10 Science

The human eye is a complex optical instrument with multiple layers and components that work together to capture and process light into images.

Parts of Human Eye and Functions:

• Cornea: Transparent front layer that refracts (bends) light rays entering eye
• Aqueous humor: Clear fluid between cornea and lens, maintains eye shape and pressure
• Iris: Colored part controlling pupil size to regulate light entry
• Pupil: Black opening whose size changes in bright/dim light
• Lens: Transparent, adjustable focusing element; changes shape to accommodate (focus) near/far objects
• Vitreous humor: Gel-like substance filling eye interior, transparent for light passage
• Retina: Light-sensitive inner layer with rods (black-white vision) and cones (color vision)
• Fovea: Center of retina with maximum cone concentration (sharpest vision)
• Blind spot: Where optic nerve exits, no photoreceptors, invisible to us
• Optic nerve: Carries visual signals from retina to brain

Accommodation is eye's ability to focus on objects at different distances by changing lens shape and strength.

Accommodation Mechanism:
Ciliary muscles contract/relax → Lens thickens/thins → Focal length changes → Image focuses on retina

Accommodation for Near vs Far Objects:

• For near objects (e.g., 25 cm): Ciliary muscles contract → Lens becomes thicker → Focal length decreases → Strong refraction converges light → Image focuses on retina
• For far objects (e.g., infinity): Ciliary muscles relax → Lens becomes thinner → Focal length increases → Weak refraction → Image focuses on retina
• Far point: Farthest point eye can see clearly (infinity, 0 power needed)
• Near point: Closest point eye can see clearly (25 cm normal for young people)
• Range of accommodation: From 25 cm to infinity in normal young eyes
• Presbyopia: Age-related loss of accommodation (lens loses elasticity after 45 years)

Power of Accommodation:

• Power = 1/f (in metres). Accommodation power = P (far point) - P (near point)
• For normal eye: P (near) = 1/0.25 = 4 D, P (far) = 1/∞ = 0 D
• Accommodation power = 4 - 0 = 4 diopters
• With age, accommodation power decreases (less elastic lens)

Vision defects occur when images don't focus precisely on retina due to eye shape or lens power errors.

Common Vision Defects:

• Myopia (Short-sightedness): Can see near but not far. Image focuses in front of retina. Caused by: eyeball too long or lens too powerful. Correction: Concave lens (negative power) diverges incoming light.
• Hyperopia (Farsightedness): Can see far but not near. Image focuses behind retina. Caused by: eyeball too short or lens too weak. Correction: Convex lens (positive power) converges light.
• Astigmatism: Blurred vision at all distances. Cornea/lens has unequal curvature (not spherical). Correction: Cylindrical lens corrects unequal curvature.
• Presbyopia: Age-related (after ~45 years). Lens loses elasticity, can't accommodate for near vision. Correction: Bifocal/progressive lenses.

Color vision in humans depends on three types of cone cells in retina responding to different wavelengths of light.

Trichromatic Vision:

• Three types of cone cells: S-cones (blue, 420 nm), M-cones (green, 530 nm), L-cones (red, 560 nm)
• Each type sensitive to different wavelengths in visible spectrum
• Color perception: Brain interprets combination of stimulation from three cone types
• Example: Red light stimulates L-cones, Green light stimulates M-cones, Blue light stimulates S-cones
• Color mixing: Red + Green = Yellow; Red + Blue = Magenta; Green + Blue = Cyan; All three = White

Color Blindness:

• Red-green color blindness: Most common (defective L or M cones). Can't distinguish red/green properly
• Blue-yellow color blindness: Rare (defective S cones)
• Complete color blindness: Extremely rare (no functional cones)
• Genetics: X-linked recessive. More common in males than females
• Testing: Ishihara color test plates to detect color blindness

Properties of Visible Light and Color:

• Visible spectrum: VIBGYOR (380-700 nm). Violet (380) to Red (700)
• Complementary colors: Pairs that mix to form white (Red-Cyan, Green-Magenta, Blue-Yellow)
• Saturation: Purity of color (fully saturated = pure hue, desaturated = grayish)
• Brightness: Intensity of light determines how bright color appears

Dispersion is splitting of white light into component colors when passing through prism or water droplets. Different wavelengths refract different amounts.

Dispersive Power of Materials:
Different wavelengths refract differently. Short wavelengths (violet) bend more than long wavelengths (red).

Spectrum Formation:

• White light dispersed by prism: Violet (most refracted), Indigo, Blue, Green, Yellow, Orange, Red (least refracted)
• Rainbow formation: Water droplets act as prisms. Sunlight enters drop, reflects internally, exits dispersed. Sun behind observer, droplets at ~42° angle.
• Spectrum wavelengths: Violet (380-420 nm), Blue (420-495 nm), Green (495-570 nm), Yellow (570-590 nm), Orange (590-620 nm), Red (620-750 nm)
• Refractive index depends on wavelength: n_violet > n_red. This causes dispersion.
• Reverse spectrum: Second rainbow from two internal reflections (reversed order, fainter)

Dispersion in Nature:

• Rainbows: Primary (1 reflection) at 42° from antisolar point, Secondary (2 reflections) at 50° with reversed colors
• Chromatic aberration: Lens defect - different colors focus at different distances. Corrected with achromatic lenses.
• Sun pillars and halos: Ice crystals disperse light creating colored phenomena
• Prism demonstrations: Newton used prisms to prove white light is mixture of colors

Rayleigh scattering is scattering of light by particles much smaller than wavelength (atoms, molecules in air). Intensity inversely proportional to λ⁴.

Rayleigh Scattering Law:
I ∝ 1/λ⁴ (Intensity inversely proportional to fourth power of wavelength)
Shorter wavelengths scatter much more than longer wavelengths

Why Sky Appears Blue:

• Blue light: λ ≈ 450 nm (shorter wavelength)
• Red light: λ ≈ 650 nm (longer wavelength)
• Scattering ratio: Blue scattered = (650/450)⁴ = ~4.4 times more than red
• Mechanism: Sunlight enters atmosphere. Blue scatters ~4x more than red. Scattered blue light reaches us from all directions = blue sky
• Not purple: Though violet λ is shorter, less in sunlight. Eyes less sensitive to violet. Eye response peaks in blue region.
• Deep blue at high altitude: Less atmosphere above = less scattering = darker blue sky

Why Sunset Appears Red:

• Longer path through atmosphere: At sunset, sunlight travels much longer distance through atmosphere
• Blue scatters away: All blue light scattered out before reaching observer. Only red/orange reaches us directly
• More scattering: Dust, pollution, water droplets increase scattering, intensifying red color
• Visible effect: Setting sun appears red/orange. Sky near horizon appears orange.
• Deep red sunsets: Pollution and dust enhance scattering, deepen color

Other Rayleigh Scattering Effects:

• Color of light scattered from dust/fog: Bluish tint when looking toward light source (Mie scattering dominates for larger particles)
• Reddening with altitude: Mountains appear clearer and more colorful at high altitude (less atmosphere to scatter)
• Moon appears reddish at horizon: Light passes through more atmosphere, blue scattered, red reaches us
• Polarization of skylight: Scattered light partially polarized (used by some insects for navigation)