Animated Diagram — The Human Eye and the Colourful World

💡 Hover over the labelled parts below to highlight them on the diagram. The eye works like a camera — cornea + lens focus light on the retina.

Cornea

Transparent front cover; provides ~2/3 of eye's focusing power (refractive index ≈ 1.376)

Iris & Pupil

Coloured muscular diaphragm; pupil regulates light entry (dilates in dark, constricts in bright)

Crystalline Lens

Biconvex, flexible; changes curvature via ciliary muscles for accommodation (f ≈ 2.5 cm)

Retina

Screen with rods (dim light) & cones (colour, 3 types: R/G/B); image forms here (inverted & real)

Optic Nerve

Transmits electrical impulses from retina to brain; exit point = blind spot (no receptors)

Ciliary Muscles

Adjust lens shape: relax → thin lens (far objects); contract → thick lens (near objects)

Object Distance: ∞ (far)

        Ciliary muscles: RELAXED · Lens: thin · Focus: ∞
      

Accommodation: The ability of the eye lens to change its focal length to focus on objects at varying distances.
Near point = 25 cm (least distance of distinct vision, D) | Far point = infinity (∞)

💡 Move the slider from far (∞) to near (25 cm). Watch how the lens thickens and ciliary muscles contract. Beyond the near point, the eye cannot accommodate further.

Myopia: Can see near objects clearly but distant objects appear blurry. Image forms in front of retina.
Cause: Elongated eyeball or excessive lens curvature.
Correction: Concave lens (diverging) of appropriate power.

Prism Angle: 60°

Dispersion: Splitting of white light into its component colours (VIBGYOR) by a prism.
Cause: Different colours have different refractive indices in glass (violet bends most, red least).
Newton's experiment: Second inverted prism recombines colours back to white.

V I B G Y O R

Wavelength increases: Violet (380nm) → Red (700nm)
Frequency decreases: Violet → Red
Speed in glass: Red (fastest) → Violet (slowest)

Rayleigh Scattering: Intensity ∝ 1/λ⁴ — shorter wavelengths scatter MUCH more.
Blue sky: Blue light (short λ) scatters most in all directions → sky appears blue when sun is overhead.

💡 At sunrise/sunset, sunlight travels through more atmosphere. Blue is scattered away completely, leaving red/orange. Danger signals use red because it scatters least and travels farthest.

💡 Click any phenomenon to see its explanation based on scattering, refraction, and dispersion.

🌈Rainbow

Cause: Dispersion + internal reflection + refraction in water droplets.
Conditions: Observer must have sun behind them, rain in front. Always appears as arc at 42° (primary) / 51° (secondary).
Primary rainbow: VIBGYOR (violet inside, red outside) — one internal reflection.
Secondary rainbow: Reversed order (red inside) — two internal reflections, fainter.

🔵Blue Sky

Cause: Rayleigh scattering of sunlight by tiny air molecules (N₂, O₂).
Scattering ∝ 1/λ⁴ — blue (λ ≈ 450 nm) scatters ~5.5× more than red (λ ≈ 700 nm).
Scattered blue light reaches our eyes from all directions → sky looks blue.

🌅Red Sunrise & Sunset

Cause: At horizon, sunlight passes through maximum atmosphere.
All shorter wavelengths (blue, violet) scatter away. Only red and orange (long λ) reach the observer directly.
Sun itself appears reddish-orange and slightly flattened (due to refraction).

⭐Twinkling of Stars

Cause: Atmospheric refraction. Starlight passes through layers of varying density (temperature → refractive index changes).
The apparent position shifts slightly and continuously → intensity fluctuates → twinkling.
Planets don't twinkle because they subtend a larger angle (extended source) — individual fluctuations average out.

🌞Advanced Sunrise & Delayed Sunset

Cause: Atmospheric refraction bends light towards the normal (denser below).
Sun appears ~2 minutes early at sunrise and stays visible ~2 minutes after it's actually below horizon.
Total extra visibility ≈ 4 minutes per day. Sun appears slightly higher than actual position.

🔴Red Danger Signals

Cause: Red light has longest wavelength → scatters the least (∝ 1/λ⁴).
Can travel the farthest through fog, dust, and smoke without being scattered away.
Used for: traffic signals, brake lights, emergency signs, tail lamps.

🌫️Tyndall Effect

Cause: Scattering of light by colloidal particles (size comparable to wavelength).
Unlike Rayleigh (molecules), Tyndall scattering depends on particle size → can scatter all colours.
Examples: Light beam through dusty room, blue colour of smoke from milk + water + laser, forest mist.

🌙Sky on Moon

Moon has no atmosphere → no molecules to scatter light.
Result: Sky appears black from the Moon even during daytime.
Stars are visible during day on the Moon (no scattered blue light to overpower them).