The Human Eye and the Colourful World

Human eye ek natural optical instrument hai jo light ko perceive karta hai. Iska structure ek camera jaisa hota hai.

Key Parts of Human Eye:

• Cornea:
• Eye ke front mein transparent bulge.
• Light sabse pehle isse enter karti hai.
• Most of the refraction light ka yahi hota hai.
• Iris:
• Cornea ke peeche ek dark muscular diaphragm.
• Pupil ke size ko control karta hai.
• Eye ka colour (blue, brown, black) iris ki wajah se hota hai.
• Pupil:
• Iris ke center mein chhota sa opening.
• Amount of light entering the eye ko regulate aur control karta hai.
• Bright light mein chhota ho jaata hai, dim light mein bada ho jaata hai.
• Crystalline Lens (Eye Lens):
• Fibrous, jelly-like material se bana hota hai.
• Focal length adjust karta hai ciliary muscles ki help se.
• Retina par real, inverted image banata hai.
• Ciliary Muscles:
• Eye lens ki curvature ko modify karte hain.
• Lens ki focal length ko adjust karte hain (accommodation).
• Retina:
• Eye ke peeche light-sensitive screen.
• Millions of light-sensitive cells (rods and cones) hote hain.
• Light energy ko electrical signals mein convert karta hai.
• Optic Nerve:
• Electrical signals ko retina se brain tak le jaata hai.
• Aqueous Humour: Cornea aur lens ke beech ka transparent fluid.
• Vitreous Humour: Lens aur retina ke beech ka jelly-like substance.

Functioning of Human Eye:

1. Light rays cornea se enter karti hain.
2. Cornea most of the refraction karta hai.
3. Iris pupil ke size ko adjust karke light amount control karta hai.
4. Light pupil se pass hokar eye lens par padti hai.
5. Eye lens light ko further refract karta hai aur retina par real, inverted image banata hai.
6. Retina ke light-sensitive cells electrical signals generate karte hain.
7. Ye signals optic nerve ke through brain tak pahunchte hain.
8. Brain signals ko interpret karta hai aur hum object ko seedha dekhte hain.

What is Accommodation?

• Eye lens ki ability ki woh apni focal length ko adjust kar sake, taaki objects ko different distances par clearly focus kar sake retina par, usse Power of Accommodation kehte hain.

How Accommodation Works:

• Distant Objects (far objects):
• Ciliary muscles relaxed hote hain.
• Eye lens thin ho jaata hai.
• Focal length increase ho jaati hai.
• Distant objects clearly dikhte hain.
• Nearby Objects (close objects):
• Ciliary muscles contract hote hain.
• Eye lens thicker ho jaata hai (curvature increase).
• Focal length decrease ho jaati hai.
• Nearby objects clearly dikhte hain.

Key Points:

• Near Point (Least Distance of Distinct Vision):
• Woh minimum distance jahan par object ko bina strain ke clearly dekha ja sake.
• Normal young adult ke liye 25 cm hota hai.
• Far Point:
• Woh farthest point jahan tak eye objects ko clearly dekh sake.
• Normal eye ke liye infinity hota hai.
• Normal Vision Range: 25 cm to infinity.

Cataract:

• Old age mein crystalline lens milky aur cloudy ho jaata hai.
• Isse partial ya complete loss of vision ho sakta hai.
• Cataract surgery se vision restore kiya ja sakta hai.

Jab eye apni power of accommodation lose kar deti hai, toh vision blurred ho jaati hai. Ye refractive defects hain, jinhe suitable lenses se correct kiya ja sakta hai.

1. Myopia (Near-sightedness)

• Defect: Person nearby objects clearly dekh sakta hai, but distant objects indistinctly dikhte hain.
• Far Point: Normal infinity ki jagah infinity se pehle aa jaata hai.
• Cause:
• Eye lens ki excessive curvature.
• Eyeball ka elongation (eye lens aur retina ke beech ka distance badh jaana).
• Image Formation: Distant object ki image retina ke saamne banti hai.
• Correction: Concave lens (diverging lens) use karte hain.
• Concave lens incoming parallel rays ko diverge karta hai, taaki image retina par ban sake.
• Lens ki power negative hoti hai.

2. Hypermetropia (Far-sightedness)

• Defect: Person distant objects clearly dekh sakta hai, but nearby objects indistinctly dikhte hain.
• Near Point: Normal 25 cm ki jagah 25 cm se door chala jaata hai.
• Cause:
• Eye lens ki focal length ka bahut zyada hona.
• Eyeball ka chhota hona (eye lens aur retina ke beech ka distance kam ho jaana).
• Image Formation: Nearby object ki image retina ke peeche banti hai.
• Correction: Convex lens (converging lens) use karte hain.
• Convex lens incoming rays ko converge karta hai, taaki image retina par ban sake.
• Lens ki power positive hoti hai.

3. Presbyopia

• Defect: Old age mein hota hai, jismein eye ki power of accommodation kam ho jaati hai.
• Symptoms: Nearby objects ko clearly dekhne mein difficulty hoti hai (near point recedes away).
• Cause:
• Ciliary muscles ka weaken hona.
• Eye lens ki flexibility ka kam hona.
• Correction:
• Convex lens use karte hain (hypermetropia jaisa).
• Agar person ko myopia aur hypermetropia dono hain, toh bifocal lenses use hote hain.
• Upper part: Concave lens (distant vision ke liye).
• Lower part: Convex lens (near vision ke liye).

Other Correction Methods:

• Contact Lenses: Directly eye par pehne jaate hain.
• Surgical Interventions: Jaise LASIK surgery.

What is a Prism?

• Ek transparent refracting medium hai jo do inclined plane surfaces se bounded hota hai.
• Ismein do triangular bases aur teen rectangular lateral surfaces hote hain.
• Angle of Prism (A): Do lateral faces ke beech ka angle.

Refraction through a Prism:

1. First Surface (Air to Glass):

• Light ray air se glass mein enter karti hai.
• Normal ki taraf bend hoti hai (denser medium mein).
• Incident ray (PE), angle of incidence (∠i), refracted ray (EF), angle of refraction (∠r).

1. Second Surface (Glass to Air):

• Light ray glass se air mein emerge karti hai.
• Normal se door bend hoti hai (rarer medium mein).
• Emergent ray (FS), angle of emergence (∠e).

Angle of Deviation (δ or D):

• Incident ray ki original direction aur emergent ray ki direction ke beech ka angle.
• Prism ki peculiar shape ki wajah se emergent ray incident ray ki direction se ek angle par bend ho jaati hai.
• Formula: \(\delta = (i + e) - A\)

Comparison with Glass Slab:

• Glass Slab: Emergent ray incident ray ke parallel hoti hai, bas laterally displaced hoti hai.
• Prism: Emergent ray incident ray ke parallel nahi hoti, balki deviated hoti hai ek angle se.

What is Dispersion?

• White light ka uske constituent colours mein split hona jab woh ek transparent medium (jaise prism) se pass karti hai, use Dispersion of Light kehte hain.

Spectrum:

• White light ke component colours ka band jo dispersion ke baad milta hai, use Spectrum kehte hain.
• White light ke spectrum mein seven colours hote hain: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).

Why Dispersion Occurs?

• Different colours of light ki wavelength alag-alag hoti hai.
• Prism ke material ka refractive index different colours ke liye different hota hai.
• Violet light ki wavelength sabse kam hoti hai, isliye woh sabse zyada bend hoti hai.
• Red light ki wavelength sabse zyada hoti hai, isliye woh sabse kam bend hoti hai.
• Is bending difference ki wajah se colours alag-alag paths par emerge hote hain aur split ho jaate hain.

Recombination of Spectrum (Newton's Experiment):

• Sir Isaac Newton ne discover kiya ki agar do identical prisms ko inverted position mein rakha jaaye, toh pehla prism white light ko disperse karta hai aur doosra prism un dispersed colours ko recombine karke dobara white light bana deta hai.
• Isse prove hota hai ki sunlight seven colours se bani hai.

Rainbow Formation:

• Rainbow ek natural spectrum hai jo sky mein rain shower ke baad dikhta hai.
• Ye sunlight ke dispersion ki wajah se hota hai jo tiny water droplets (small prisms ki tarah act karte hain) karte hain atmosphere mein.
• Process:

1. Sunlight water droplet mein enter karti hai aur refract hoti hai (aur disperse bhi).
2. Dispersed light droplet ke inner surface se internally reflect hoti hai.
3. Reflected light droplet se bahar nikalte waqt dobara refract hoti hai.
4. Dispersion aur internal reflection ki wajah se different colours observer ki eye tak pahunchte hain.

• Rainbow hamesha Sun ke opposite direction mein banta hai.

What is Atmospheric Refraction?

• Earth ke atmosphere mein light ka refraction, jismein refractive index continuously change hota rehta hai (temperature aur density variations ki wajah se).

Phenomena due to Atmospheric Refraction:

1. Twinkling of Stars:

• Stars point-sized light sources hote hain kyunki woh bahut door hain.
• Starlight atmosphere mein enter karti hai aur continuously refract hoti hai (atmosphere ki changing density ki wajah se).
• Is continuous refraction ki wajah se star ki apparent position fluctuate karti hai.
• Light ki amount jo eye tak pahunchti hai woh flicker karti hai, jiski wajah se stars twinkle karte hue dikhte hain (kabhi bright, kabhi faint).
• Planets twinkle nahi karte kyunki woh Earth ke bahut kareeb hain aur extended sources ki tarah dikhte hain. Unse aane wali light rays ka total variation average out ho jaata hai.

1. Advance Sunrise and Delayed Sunset:

• Atmospheric refraction ki wajah se Sun actual sunrise se approx 2 minutes pehle dikhna shuru ho jaata hai.
• Aur actual sunset ke approx 2 minutes baad tak dikhta rehta hai.
• Jab Sun horizon ke neeche hota hai, tab bhi uski light atmosphere se refract hokar hamari eye tak pahunch jaati hai, jisse uski apparent position thodi upar dikhti hai.
• Isse din ka duration 4 minutes badh jaata hai.

1. Apparent Flattening of Sun's Disc:

• Sunrise aur sunset ke time Sun ka disc thoda chipta (flattened) dikhta hai, ye bhi atmospheric refraction ki wajah se hota hai.

1. Wavering of Objects over Hot Air:

• Hot air (jaise fire ke upar) cooler air se lighter aur less dense hoti hai, iska refractive index bhi kam hota hai.
• Hot air ki physical conditions stationary nahi hoti, jiski wajah se uske through dikhne wale objects waver ya flicker karte hain.

What is Scattering of Light?

• Jab light minute particles (jaise dust, smoke, air molecules) se takrati hai, toh woh alag-alag directions mein fail jaati hai. Is phenomenon ko Scattering of Light kehte hain.
• Scattered light ka colour scattering particles ke size par depend karta hai.

1. Tyndall Effect:

• Jab light beam colloidal solution ya fine particles wale medium se pass karti hai, toh uska path visible ho jaata hai.
• Ye light ke scattering ki wajah se hota hai.
• Examples:
• Sunlight ka smoke-filled room mein chote hole se enter karna.
• Dense forest ke canopy se sunlight ka pass hona (mist mein water droplets).

2. Why is the Colour of the Clear Sky Blue?

• Earth ka atmosphere air molecules aur fine particles ka heterogeneous mixture hai.
• Ye particles visible light ki wavelength se chote hote hain.
• Ye particles shorter wavelengths (blue light) ko longer wavelengths (red light) se zyada effectively scatter karte hain.
• Red light ki wavelength blue light se approx 1.8 times zyada hoti hai, isliye blue light zyada scatter hoti hai.
• Jab sunlight atmosphere se pass karti hai, toh blue colour zyada scatter hokar hamari eyes tak pahunchta hai, isliye sky blue dikhta hai.
• Agar Earth par atmosphere nahi hota, toh sky dark dikhta (jaise astronauts ko space mein dikhta hai) kyunki koi scattering nahi hoti.

3. Colour of Sun at Sunrise and Sunset:

• Sunrise aur sunset ke time Sun reddish dikhta hai.
• Is time Sun horizon ke paas hota hai, aur sunlight ko atmosphere ke zyada thick layer se travel karna padta hai.
• Zyada distance travel karne ki wajah se blue light aur shorter wavelengths bahut zyada scatter ho jaati hain aur hamari eyes tak nahi pahunch paati.
• Sirf red light (longest wavelength, least scattered) hi hamari eyes tak pahunch paati hai, isliye Sun reddish dikhta hai.

4. Danger Signals are Red:

• Danger signals red colour ke hote hain kyunki red light fog ya smoke se sabse kam scatter hoti hai.
• Isliye red light door se bhi clearly dikhti hai, bina apni intensity khoe.