Light – Reflection and Refraction: Class 10 Science Chapter 9 Hinglish mein

Amavasya ki raat mein light chali jaaye, to tumhara kamra jaise gayab ho jaata hai. Bed aur almirah wahin hain, phir bhi tum unhe dekh nahi paate. Candle jalao aur woh wapas dikhne lagte hain. Hum koi cheez tabhi dekhte hain jab usse light chalkar hamari aankhon tak aati hai. Sun, bulb ya candle ki flame apni light khud banate hain; book, deewar ya chehra apne upar padne wali light ka kuch hissa wapas bhejte hain. Light saaf glass aur paani ke aar-paar bhi ja sakti hai, isiliye window se tum sadak dekh paate ho.

Dhool bhare kamre mein torch ki beam dekho, ya parde ke gap se aati sunlight ki dhaar: woh seedhi chalti hai. Chhota bulb tez kinaron wali shadow banata hai, kyunki light kisi object ke around mudkar uske peeche ki jagah mein nahi jaati. Is seedhe raaste ko hum arrow wali line se dikhate hain aur use ray kehte hain. Is chapter mein mirrors, lenses aur light ke mudne ko samjhane ke liye humein bas rays chahiye. Yahi ideas agle chapter mein human eye aur rainbow tak le jaate hain.

9.1 Reflection of Light

Smooth, chamakdar surfaces jaise mirror, polished steel plate ya thehra hua paani, un tak pahunchne wali zyadatar light ko wapas bhej dete hain. Reflection ke do rules tum pichhli classes mein padh chuke ho. Yeh har surface par lagte hain, chahe flat ho ya curved, bas angles normal se naapne hain: woh line jo ray ke takraane wale point par surface ke right angle par kheenchi jaati hai.

To plane mirror jo image deta hai woh virtual hoti hai (glass ke peeche us jagah par asal mein koi light nahi pahunchti), erect hoti hai aur object ke same size ki hoti hai. Woh glass ke peeche theek utni gehrai par hoti hai jitna object aage khada hai, aur woh laterally inverted hoti hai: tumhara right haath image ka left haath dikhta hai. Isiliye ambulance ke bonnet par AMBULANCE shabd ulta likha hota hai. Aage wala driver use rear-view mirror mein seedha padh leta hai.

Jab mirror curved ho to kya hota hai? Aage padhne se pehle neeche wali activity karke dekho.

Katori ki dono sides curved mirrors hain. Sabse useful curved mirrors ki reflecting surface ek sphere ka hissa hoti hai, isliye unhe spherical mirrors kehte hain.

9.2 Spherical Mirrors

Ek khokhli glass ki ball socho, jis par silver polish ho aur jise tukdon mein kaata gaya ho. Har tukda ek spherical mirror hai. Agar reflecting face andar wala, dhansa hua face hai, jo sphere ke centre ki taraf hai, to woh tukda concave mirror (अवतल दर्पण) hai. Agar reflecting face bahar ki taraf ubhra hai, to woh convex mirror (उत्तल दर्पण) hai. Steel ke chammach ka andar wala hissa lagbhag concave aur peeche wala lagbhag convex hota hai. Diagrams mein hum spherical mirror ko curved line se banate hain aur uski peeth, yaani reflect na karne wali side, par hatching karte hain.

Kuch naam humein in mirrors ko theek-theek describe karne dete hain (Fig. 9.2).

Sunlight ek doosre ke parallel rays ki tarah aati hai. Jab principal axis ke parallel rays concave mirror par padti hain, to sab reflect hokar axis ke ek hi point se guzarti hain. Jab woh convex mirror par padti hain to phail jaati hain, lekin peeche trace karne par lagta hai ki woh mirror ke peeche ek hi point se aa rahi hain (Fig. 9.2 ke aakhri do steps).

Reflecting surface ki chaudai uska aperture hai. Hum sirf aise mirrors lete hain jinka aperture unke radius of curvature se chhota ho. Inke liye focus pole aur centre of curvature ke theek beech mein hota hai, isliye radius of curvature focal length ka do guna hota hai:

9.2.1 Spherical Mirrors se Image Formation

Plane mirror hamesha ek hi tarah ki image deta hai. Concave mirror aisa nahi karta: uski image mirror ke saamne ya peeche ban sakti hai, aur badi ya chhoti, seedhi ya ulti ho sakti hai, yeh is par depend karta hai ki object kahan hai. Do shabd har image ko describe karte hain.

9.2.2 Ray Diagrams se Spherical Mirrors ki Images Dikhana

Object ka har point har direction mein rays bhejta hai, aur mirror har point ki image banata hai. Object ke tip ki image kahan hai, yeh jaanne ke liye humein saari rays nahi chahiye. Hum aisi do rays follow karte hain jinka reflection ke baad ka raasta hum pehle se bata sakte hain, aur dekhte hain ki woh kahan cross karti hain. Chaar rays ko follow karna khaas taur par aasaan hai (Fig. 9.3).

1. Principal axis ke parallel ray reflect hokar F se guzarti hai (concave mirror), ya aise reflect hoti hai ki F se aati lagti hai (convex mirror).
2. F se guzarti ray (concave mirror), ya F ki taraf jaati ray (convex mirror), reflect hokar principal axis ke parallel chalti hai. Yeh pehla rule ulta chalaane jaisa hai.
3. C se guzarti ray (concave mirror), ya C ki taraf jaati ray (convex mirror), mirror se normal ke along milti hai aur usi line par wapas lautti hai.
4. Pole par padne wali ray principal axis ki doosri taraf barabar angle par reflect hoti hai, kyunki P par axis hi normal hai.

Inmein se koi bhi naya law nahi hai: har ek bas suvidha wale point par lagaya gaya law of reflection hai. Image dhoondhne ke liye inmein se koi bhi do use karo; teesri ray check karne ke kaam aati hai.

(a) Concave mirror se image formation

Table 9.3 ki har object position ke liye Fig. 9.4 parallel ray aur F se guzarti (ya F se aati) ray use karta hai. Ise step by step dekho aur pattern par dhyaan do. Jaise-jaise object F ki taraf aata hai, real image mirror se door bhaagti hai aur badi hoti hai. Jaise hi object F ke andar aata hai, reflected rays saamne nahi milti, aur image kood kar mirror ke peeche chali jaati hai.

Explorer mein concave mirror ke liye har position button dabao aur status line ko Table 9.3 se milao. Phir object ko dheere-dheere F ke aar-paar khiskao aur dekho kaise image ek taraf infinity mein gayab hoti hai aur mirror ke peeche phir dikhti hai.

Concave mirrors ke uses. Har use Table 9.3 ki ek row se milta hai:

• Solar cookers aur solar furnaces: ek bada concave reflector sunlight ko apne focus par laata hai, jahan concentrated energy khaana pakaati hai ya metal bhi pighla deti hai (pehli row).
• Dentists ke mirrors: daant ke paas pakda chhota concave mirror uska magnified view dikhata hai.
• Shaving aur make-up mirrors: tumhara chehra F se zyada paas hota hai, isliye tumhe badi, seedhi image dikhti hai (aakhri row).
• Torches, searchlights aur vehicle headlights: bulb F par hota hai, isliye reflected light ek strong parallel beam ban kar nikalti hai (F par wali row).

(b) Convex mirror se image formation

Convex mirror light ko phailata hai, isliye reflected rays uske saamne kabhi nahi milti. Object kahin bhi ho, rays sirf mirror ke peeche P aur F ke beech ke ek point se aati lagti hain. Isliye image hamesha virtual, seedhi aur object se chhoti hoti hai. Sirf do cases ke liye diagram chahiye: bahut door ka object aur normal doori par rakha object (Fig. 9.6).

Convex mirrors ke uses. Bus ya scooter ka wing mirror dekho: woh ubhra hua hai. Driver ko usse do cheezein chahiye. Pehli, har doori par seedhi image, taaki peeche ka traffic kabhi ulta na dikhe. Doosri, peeche ki sadak ka wide view: ubhaar same size ke flat mirror se kahin zyada wide angle se light ikattha karta hai. Yahi mirrors ghat roads ke tez modon aur dukaanon ke kono par bhi lage hote hain. Dikkat yeh hai ki vehicles chhote dikhte hain, isliye woh asal se zyada door lagte hain.

9.2.3 Spherical Mirrors se Reflection ke liye Sign Convention

Mirrors ke liye formula use karne ke liye humein batana padta hai ki har doori kis taraf hai. Hum New Cartesian sign convention use karte hain. Isme principal axis ko graph ka x-axis maana jaata hai, aur origin pole par hota hai (Fig. 9.8):

1. Object ko mirror ke left mein rakho, taaki light left se right mirror ki taraf chale.
2. Principal axis ke along har doori pole se naapo.
3. Pole ke right mein (+x ki taraf) naapi dooriyan positive hain; left mein (−x ki taraf) naapi dooriyan negative hain.
4. Principal axis se upar (+y) naapi heights positive hain.
5. Principal axis se neeche (−y) naapi heights negative hain.

Isse do shortcuts milte hain. Object distance $u$ hamesha negative hota hai, kyunki object hamesha left mein hota hai. Concave mirror ka focus uske saamne hota hai, isliye uska $f$ (aur $R$) negative hai; convex mirror ka focus uske peeche hota hai, isliye uska $f$ positive hai.

9.2.4 Mirror Formula aur Magnification

Teen lengths, sab pole se naapi hui, ek mirror set-up ko describe karti hain: object distance $u$, image distance $v$, aur focus ke liye $f$. Mirror formula inhe jodta hai:

Ek hi formula har case cover karta hai, concave ho ya convex, object paas ho ya door, bas har quantity apne sign ke saath daalni hai. Sign kabhi guess mat karo: convention se tay karo, daalo, aur algebra ko answer ka sign batane do.

Magnification. Magnification (आवर्धन) $m$ batata hai ki image object se kitne guna lambi hai. Jahan $h$ object height hai aur $h^{\mathrm{\prime }}$ image height:

Ise dono dooriyon se bhi nikaala ja sakta hai:

Object axis par upar ki taraf khada hai, isliye $h$ positive hai. Tab $m$ ka sign image ka nature batata hai. Negative $m$ ka matlab hai ki image axis ke neeche latki hai: woh inverted aur real hai. Positive $m$ ka matlab hai ki woh seedhi aur virtual hai. $m$ ka size baaki batata hai: $\mathrm{\mid }m\mathrm{\mid }>1$ enlarged, $\mathrm{\mid }m\mathrm{\mid }=1$ same size, $\mathrm{\mid }m\mathrm{\mid }<1$ diminished.

9.3 Refraction of Light

Saaf swimming pool mein neeche dekho to tala asal se zyada paas lagta hai; jo log kam gehra paani samajhkar koodte hain, unhe jhatka lagta hai. Nimbu-paani ke glass mein straw surface par tooti hui dikhti hai. Glass ka paperweight apne neeche ke shabdon ko upar utha hua dikhata hai, aur gol bowl mein machhli side se dekhne par badi lagti hai.

Har case mein light tumhari aankh tak pahunchne se pehle ek transparent material se doosre mein jaati hai, paani ya glass se hawa mein. Straw ke paani ke andar wale hisse se aati light surface se nayi direction mein nikalti hai. Tumhara dimaag maanta hai ki light seedhi lines mein chalti hai, isliye woh straw ke us hisse ko galat jagah par dikhata hai. Yeh effect kuch materials ke pairs ke liye doosron se zyada strong hota hai: cooking oil mein straw alag hi had tak mudi dikhti hai. Direction ke is badlaav ko refraction (अपवर्तन) kehte hain.

9.3.1 Rectangular Glass Slab se Refraction

Refracted ray trace karne ke liye rectangular glass slab sabse simple jagah hai, kyunki uske do lambe faces flat aur parallel hote hain. Classic method mein pins use hoti hain.

Dono faces par mudna barabar aur ulta hota hai. Faces parallel hain, isliye doosra face direction mein pehle face ka kiya hua theek-theek mita deta hai. Isliye ray apni original direction mein nikalti hai, bas thodi si ek taraf khiski hui. Slab jitna mota aur ray jitni tirchi, yeh lateral (side wala) shift utna bada. Jo ray slab se normal ke along milti hai, woh bina mude seedhi nikal jaati hai.

Light mudti hi kyun hai? Ek medium se doosre mein jaate waqt uski speed badalti hai: glass mein light air se dheere chalti hai. Socho, sainikon ki ek row sadak se ret par tirchi march kar rahi hai. Row ka jo end pehle ret par pahunchta hai woh pehle dheema hota hai, isliye poori row ghoomkar nayi direction mein mud jaati hai. Experiments dikhate hain ki refraction do laws follow karta hai.

Is constant ka ek naam hai, refractive index, aur agla section isi ke baare mein hai.

9.3.2 Refractive Index

Media ke kuch pairs light ko bahut modte hain aur kuch bas thoda sa. Refractive index (अपवर्तनांक) ise ek number deta hai, aur yeh number light ki speed se juda hai. Khaali space mein light se tez kuch nahi, lagbhag $3\times {10}^8\mathrm{m}\mathrm{/}\mathrm{s}$. Air use sirf lagbhag 0.03 % dheema karti hai, lekin paani aur glass use ek-chauthai se ek-tihaai tak dheema kar dete hain.

Maan lo light medium 1 se, jahan uski speed $v_1$ hai, medium 2 mein jaati hai, jahan uski speed $v_2$ hai. Pehli speed ko doosri se divide karne par $n_{21}$ milta hai, yaani medium 1 ke relative medium 2 ka refractive index:

Media ko badal do to ratio ulta ho jaata hai:

Jab pehla medium vacuum ho (air kaafi kareeb hai, isliye use bhi waise hi lete hain), to ratio medium ka absolute refractive index hota hai, jise aksar bas uska refractive index $n_m$ kehte hain. Jahan $c$ vacuum mein light ki speed hai aur $v$ medium mein uski speed:

Paani ke liye $n=1.33$: jitne time mein light air mein 1.33 m chalti hai, utne mein paani mein sirf 1 m chalti hai. Crown glass ka $n=1.52$ hai. Aisi values ratne ki zaroorat nahi, lekin unhe use karna aana chahiye. Fig. 9.11 ek hi ray ko teen materials mein bhejta hai: refractive index jitna bada, ray normal ki taraf utni tezi se mudti hai.

9.3.3 Spherical Lenses se Refraction

Ghadi theek karne wala ek aankh mein chhota magnifier lagata hai; reading glasses print ko bada dikhate hain; phone camera mein chhote-chhote lenses ka ek stack chhupa hota hai. Lens (लेंस) glass ya plastic ka ek saaf tukda hai jiska kam se kam ek face sphere ke hisse jaisa curved hota hai; doosra face flat bhi ho sakta hai.

Jo lens kinaron se zyada beech mein mota ho, woh convex lens hai. Woh parallel rays ko ikattha karta hai, isliye use converging lens bhi kehte hain. Jo lens beech mein patla ho, woh concave lens hai; woh parallel rays ko phailata hai aur use diverging lens kehte hain. Is chapter mein 'convex lens' ka matlab hai aisa lens jiske dono faces bahar ki taraf ubhre hon, aur 'concave lens' ka matlab jiske dono faces andar ki taraf mude hon.

Har curved face ek sphere ka hissa hai, isliye lens ke do centres of curvature hote hain, $C_1$ aur $C_2$. Inse guzarti line principal axis hai. Is axis par lens ka central point uska optical centre O hai. Lens ka diameter uska aperture hai. Hum sirf chhote aperture wale thin lenses padhte hain, jinke dono centres of curvature O se barabar door hote hain.

Principal axis ke parallel rays convex lens se guzarne ke baad doosri side axis ke ek point par milti hain: yeh principal focus hai. Concave lens ke baad woh aise phailti hain jaise us side axis ke ek point se aa rahi hon jahan se woh andar aayi thi. Light lens mein kisi bhi side se aa sakti hai, isliye har lens ke do principal foci hote hain, $F_1$ aur $F_2$, har side ek, aur O se barabar door. Yeh doori lens ki focal length $f$ hai.

9.3.4 Lenses se Image Formation

Concave mirror ki tarah, convex lens bhi object ki position ke hisaab se alag-alag tarah ki images banata hai. Hum unhe Activity 9.3 jaise bench experiment se dhoondh sakte hain.

9.3.5 Ray Diagrams se Lenses mein Image Formation

Lenses ke ray diagrams bilkul mirrors jaise hi kaam karte hain, bas rays wapas lautne ki jagah aar-paar jaati hain. Thin lens ke liye hum saara mudna lens ki central line par dikhate hain. In teen rays mein se koi bhi do image dhoondh leti hain (Fig. 9.13):

1. Principal axis ke parallel ray convex lens ke baad $F_2$ se guzarti hai, ya concave lens ke baad aise phailti hai jaise $F_1$ se aa rahi ho.
2. Convex lens ke $F_1$ se guzarti ray, ya concave lens ke $F_2$ ki taraf jaati ray, principal axis ke parallel nikalti hai.
3. Optical centre O se guzarti ray bina mude seedhi chalti hai.

Fig. 9.14 parallel ray aur O se guzarti ray ko Table 9.8 aur Table 9.9 ki har row par lagata hai. Phir lens explorer use karo: object ko $2F_1$ aur $F_1$ ke aar-paar drag karo aur dekho kaise image ki side aur character badalte hain.

9.3.6 Spherical Lenses ke liye Sign Convention

Lenses mein bhi mirrors wala New Cartesian convention lagta hai, bas ek badlaav ke saath: har doori pole ki jagah optical centre O se naapi jaati hai. Light ab bhi left se aati hai, isliye $u$ negative hai. Convex lens ke liye jo focus kaam ka hai, $F_2$, woh right mein hai, isliye uski focal length positive hai; concave lens ki focal length negative hoti hai. Axis ke upar ki heights positive aur neeche ki negative hain, pehle jaisa.

9.3.7 Lens Formula aur Magnification

Lens formula lens ke liye $u$, $v$ aur $f$ ko jodta hai. Minus sign par dhyaan do, jo mirror formula se alag hai:

Mirrors ki tarah, ek hi formula dono tarah ke lens aur har object position ke liye kaam karta hai, bas signs convention se aane chahiye. Magnification bilkul mirrors ki tarah define hota hai:

Lekin dooriyon ke saath iske relation mein minus sign nahi hai:

Yeh fark kyun? Lens se bani real image right mein banti hai ($v$ positive) jabki object left mein hota hai ($u$ negative), isliye real, inverted image ke liye $v\mathrm{/}u$ pehle se negative hai. Mirror mein real image object wali side par hi banti hai, isliye $m$ ko negative banane ke liye ek extra minus sign chahiye. Dono ke liye matlab same hai: negative $m$ matlab real aur inverted, positive $m$ matlab virtual aur erect.

9.3.8 Lens ki Power

Dhoop mein ek mota aur ek patla magnifying glass pakdo. Mota wala light ko apne bahut paas ek point par laata hai, kyunki woh rays ko zyada tezi se modta hai. Concave lenses bhi aise hi hain: focal length jitni chhoti, woh light ko utna zyada phailate hain. Lens ki power (क्षमता) light ko modne ki uski ability ko naapti hai. Yeh focal length ka reciprocal hai:

$f$ metres mein ho to power dioptres (D) mein aati hai, aur $1\mathrm{D}=1{\mathrm{m}}^{-1}$: jo lens parallel light ko ek metre door focus kare, uski power ek dioptre hai. $f$ ke sign ke hisaab se convex lens ki power positive aur concave lens ki negative hoti hai (Fig. 9.16).

Opticians prescriptions dioptres mein likhte hain. $+1.25\mathrm{D}$ ke prescription ka matlab hai $f=1\mathrm{/}1.25=+0.80\mathrm{m}$ wala convex lens. $-4.0\mathrm{D}$ ka matlab hai $f=-0.25\mathrm{m}$, yaani $25\mathrm{c}\mathrm{m}$, wala concave lens.