Reflection ke Niyam (Roshni Kis Taraf Lautegi?)
Sheeshe se dhoop ka dhabba deewar pe kahan girega, kya pehle se bata sakte hain? Haan, do niyam se! 🪞☀️
🧭 Is Post Mein
📖 Kahani: Deewar pe Naachta Dhoop ka Dhabba
Ravivaar ki subah thi. Aangan mein tez dhoop aa rahi thi. Golu ke haath mein mummy ka chhota sa sheesha tha, aur woh dhoop ka ek chamakta dhabba saamne ki chhaaon wali deewar pe naacha raha tha.
“Didi, dekho! Main sheesha thoda sa ghumata hoon toh dhabba poori deewar pe bhaag jaata hai!” Golu hansa.
Priya ne chunauti di: “Achha? Toh pehle se batao, dhabba us khidki ke theek upar girega, iske liye sheesha kaise pakdoge?” Golu atak gaya. Kabhi dhabba upar jaata, kabhi neeche.
Rohan bhaiya bole: “Andaaze se nahi, niyam se karo. Roshni sheeshe se takra kar jaise-taise nahi lautti. Woh ek pakke niyam ke hisaab se lautti hai. Niyam jaan lo, toh dhabba kahan girega, yeh pehle hi bata sakte ho.”
Pichhle posts mein humne dekha ki mirror kaise image banate hain aur kahan use hote hain. Aaj seekhenge woh Reflection ke Niyam (Laws of Reflection), jinke hisaab se har mirror roshni lautata hai. 🔍
🎯 Is Post Mein Kya Seekhenge
- Roshni ko ray (teer wali seedhi line) se kaise dikhate hain
- Incident ray, reflected ray, point of incidence aur normal kya hote hain
- Angle of incidence (i) aur angle of reflection (r) kaise naapte hain
- Reflection ka pehla niyam: i = r
- Reflection ka doosra niyam: incident ray, normal aur reflected ray ek hi samtal (plane) mein
- Normal ke saath aayi roshni kya karti hai, aur kya yeh niyam curved mirror pe bhi chalte hain
🧱 Basics: Pehle Yeh Aana Chahiye
1. Roshni seedhi line mein chalti hai. Isliye hum roshni ke raaste ko ek seedhi line aur uspe teer (arrow) se dikhate hain. Isko ray (kiran) kehte hain. Teer batata hai ki roshni kis taraf ja rahi hai.
2. Reflection: chamakdaar sateh se takra kar roshni ka wapas lautna.
3. Angle (kon) naapna: do lines ke beech ka khulaapan. Isse protractor (chaanda) se degree (°) mein naapte hain. 90° ka kon “seedha khada” kon hota hai, jaise kitaab ka kona.
Mirror ke prakaar bhool gaye? Pichhla post dekh lo: Concave aur Convex Mirror ke Upyog.
💡 Concept: Do Kirnein, Ek Normal, Do Kon
Step 1: Incident ray. Jo kiran mirror ki taraf aa kar us pe girti hai, use incident ray (aapatit kiran) kehte hain. Socho, yeh “jaane wali” kiran hai.
Step 2: Point of incidence. Mirror pe woh point jahan incident ray takrati hai. Ise aksar O se dikhate hain.
Step 3: Reflected ray. Jo kiran mirror se takra kar wapas aati hai, woh reflected ray (paravartit kiran) hai. Yeh “lautne wali” kiran hai.
Step 4: Normal, sabse zaroori line! Point O se mirror pe 90° ka kon banate hue ek line kheencho. Yeh normal (abhilamb) hai. Yeh koi kiran nahi, sirf naapne ke liye kheenchi gayi ek madadgaar line hai, isliye ise aksar dotted (tooti) line se banate hain.
🔸 Angle of reflection (r): reflected ray aur normal ke beech ka kon.
Dhyaan do: dono kon normal se naapte hain, mirror ki sateh se nahi!
Step 5: Pehla niyam (Law 1). Jab hum dhyaan se naapte hain, toh har baar milta hai:
Yaani roshni jitne kon pe normal se aa kar girti hai, utne hi kon pe normal ki doosri taraf laut jaati hai. Jaise koi ball deewar se takra kar lagbhag usi tirchhepan se doosri taraf lautti hai.
Step 6: Doosra niyam (Law 2). Incident ray, point of incidence pe normal, aur reflected ray, teeno ek hi samtal (plane) mein hote hain. “Samtal” ko ek seedhe, sapaat kaagaz ki tarah socho. Teeno lines ek hi kaagaz pe bichhi rehti hain. Reflected ray kabhi us kaagaz se bahar, upar ya neeche nahi nikal jaati.
Step 7: Khaas case, roshni normal ke saath aaye toh? Agar roshni mirror pe bilkul seedhi (normal ke saath-saath) gire, toh i = 0°, isliye r = 0°. Roshni usi raaste se wapas laut aati hai. Reflection hota hai, bas roshni seedhi peeche lautti hai.
Step 8: Kya curved mirror pe bhi? Haan! Yeh dono niyam plane, concave aur convex, sab mirrors pe laagu hote hain. Curved mirror mein har point pe normal ki disha alag hoti hai, isliye poori roshni ka pattern alag dikhta hai. Iske baare mein agle post mein vistaar se padhenge.
🖼️ Diagrams: Picture se Samjho
🔐 Deep Concept: Normal Itna Zaroori Kyun Hai?
Raaz 1: Mirror se kon kyun nahi naapte? Curved mirror ki sateh har jagah mudi hoti hai, toh “sateh se kon” ka koi ek pakka matlab nahi banta. Lekin har point pe normal ek saaf, pakki line hai. Isliye scientists ne tay kiya ki kon hamesha normal se naapenge. Isi se ek hi niyam har mirror pe chal jaata hai.
Raaz 2: Incident aur reflected ray ke beech ka kon. Kyunki i = r, dono kirnon ke beech ka poora kon i + r = 2i hota hai. Jaise i = 35° ho, toh dono kirnon ke beech 70°.
Raaz 3: Curved mirror pe normal kaise? Concave ya convex mirror ek bade gole (sphere) ka hissa hai. Kisi bhi point pe normal woh line hai jo us point ko gole ke kendra (centre) se jodti hai. Har point pe yeh disha badal jaati hai. Har kiran apne point pe i = r hi maanti hai, par normal alag-alag hone se kirnein milti (concave) ya phailti (convex) hain.
Raaz 4: Khurdari deewar pe chehra kyun nahi dikhta? Deewar bhi roshni lautati hai, aur har chhoti kiran i = r maanti hai. Par khurdari sateh ke chhote-chhote tukde alag-alag taraf mude hote hain, toh unke normal bhi alag. Isliye lauti kirnein har taraf bikhar jaati hain (diffused reflection) aur image nahi banta. Chikni sateh pe sab normal ek jaise, toh image saaf banta hai (regular reflection).
✏️ Examples: Niyam Lagao, Kon Nikaalo
Pehla niyam: i = r.
Normal mirror se 90° pe hai, toh i = 90° − 20° = 70°.
r = 40° (normal se), toh sateh se 90° − 40° = 50°.
i + r = 100° aur i = r, toh i = 100° ÷ 2 = 50°.
Kiran normal ke saath hai, isliye usi raaste se wapas lautegi.
Niyam har mirror pe chalte hain; bas normal us point pe gole ke kendra ki taraf hota hai.
⚖️ Difference Table: Incident Ray vs Reflected Ray
| Baat | Incident ray | Reflected ray |
|---|---|---|
| Kya hai? | Mirror pe girne wali kiran | Mirror se lautne wali kiran |
| Teer ki disha | Mirror ki taraf (point O ki taraf) | Mirror se door (O se bahar) |
| Normal se kon | Angle of incidence (i) | Angle of reflection (r) |
| Kaun kis pe nirbhar? | Hum tay karte hain (torch kaise pakdi) | Incident ray se tay hoti hai (r = i) |
| Normal ke saath aaye toh | i = 0° | r = 0°, usi raaste se wapas |
🚫 Misconceptions: Log aisa sochte hain… lekin
❌ “Angle of incidence mirror ki sateh aur kiran ke beech hota hai.”
✅ Sach: yeh kiran aur normal ke beech hota hai. Sateh wala kon = 90° − i.
❌ “Normal bhi ek roshni ki kiran hai.”
✅ Sach: normal sirf naapne ke liye kheenchi gayi kalpanik madadgaar line hai. Isliye use dotted banate hain aur us pe teer nahi lagate.
❌ “Roshni seedhi mirror pe gire toh reflection nahi hota.”
✅ Sach: reflection hota hai; i = 0°, r = 0°, aur roshni usi raaste se wapas aati hai.
❌ “Yeh niyam sirf plane mirror ke liye hain.”
✅ Sach: concave, convex, sab mirrors pe har kiran i = r maanti hai.
❌ “Khurdari deewar reflection ke niyam nahi maanti, isliye chehra nahi dikhta.”
✅ Sach: har chhoti kiran niyam maanti hai; bas sateh ke tukdon ke normal alag-alag hone se kirnein bikhar jaati hain.
🔤 Key Terms (Exam mein English shabd hi likhna)
❓ FAQs
1. Normal hamesha mirror pe seedha khada (90°) hi kyun kheenchte hain?
Kyunki 90° wali line har point pe ek hi hoti hai, toh sab log ek hi tarah naapenge aur ek jaisa jawaab aayega. Isse niyam saaf aur pakka ban jaata hai.
2. Activity mein i aur r kabhi-kabhi 1–2° alag kyun aate hain?
Yeh naapne ki chhoti galti hai: beam thoda mota hota hai, line kheenchne mein ya protractor rakhne mein thoda farq aa jaata hai. Dhyaan se karein toh dono lagbhag barabar aate hain.
3. i = 90° ho sakta hai kya?
i = 90° ka matlab kiran mirror ki sateh ke saath-saath chal rahi hai, woh mirror pe girti hi nahi. Isliye reflection ke liye i hamesha 0° se 90° ke beech (90° se kam) hota hai.
4. Doosra niyam roz ki zindagi mein kaise dikhta hai?
Jab aap sheeshe se dhoop ka dhabba deewar pe daalte ho, toh dhabba wahin girta hai jahan incident ray aur normal wale samtal mein reflected ray jaati hai. Isi se aap dhabbe ki jagah pehle se bata sakte ho.
5. Kya paani ki sateh bhi yeh niyam maanti hai?
Haan. Shaant paani ek plane mirror ki tarah kaam karta hai aur i = r maanta hai. Isliye taalaab mein pedon ka saaf ulta pratibimb dikhta hai.
🧪 Activity (NCERT): Kon Naapo, Niyam Dhoondo
Saamaan: stand wala chhota plane mirror (ya kitaab ke sahaare khada mirror), torch, kanghi (comb), kaala kaagaz, paper clip, safed kaagaz, ek moti chart sheet, pencil, scale aur protractor.
Bhaag 1: Pehla niyam
Bhaag 2: Doosra niyam
Kya dikhega:
🔹 Kiran normal ke saath giraane pe usi raaste se wapas aati hai (i = r = 0°).
🔹 Sheet sapaat ho toh reflected kiran bahar wale hisse pe dikhti hai; modte hi gayab, seedha karte hi phir dikhti hai.
Nishkarsh: (1) Angle of incidence = Angle of reflection. (2) Incident ray, normal aur reflected ray ek hi samtal mein hote hain. Mudi hui sheet ek naya samtal ban jaati hai, isliye kiran us pe nahi padti.
Yeh activity NCERT ki Class 8 Science book Curiosity (Chapter 10) par aadharit hai, aur yahan apne shabdon mein samjhayi gayi hai.
🏠 Mini Experiments (Ghar pe)
Isse kya pata chala? Mirror ghumane se normal ki disha badalti hai, toh i aur r dono badalte hain, aur reflected kiran ki disha badal jaati hai. Deewar door ho toh chhota sa ghumaav bhi dhabbe ko kaafi door khiska deta hai.
Farsh pe deewar ke saamne chalk se deewar pe 90° ki ek line (normal) banao. Ek ball ko tirchha ludhka kar us line wali jagah deewar se takrao. Dekho ball lagbhag utne hi tirchhepan se doosri taraf lautti hai. (Yeh sirf ek tulna hai; ball ki ghisaai aur ghoomne se thoda farq aa sakta hai, par roshni ke liye i = r pakka niyam hai.)
Copy pe ek seedhi line (mirror) kheencho, beech mein point O lo aur protractor se normal banao. Normal se 30° pe incident ray kheencho, phir doosri taraf 30° pe reflected ray. Ab 45° aur 60° ke saath dohrao. Dekho, i badhne pe reflected ray mirror ke kareeb aati jaati hai.
🧠 Think Like a Scientist
Sawaal: Golu ne socha: “Chhote kon pe toh i = r aata hai, par bahut tirchhi kiran (jaise 70°) pe shayad niyam toot jaaye?” Kaise pata karein?
Scientist ka tareeka dekho
1. Observe (dekho): 20° aur 35° pe r lagbhag i ke barabar aaya.
2. Guess (anumaan): Golu ka anumaan: bahut tirchhe kon pe r alag aa sakta hai.
3. Test (jaanch): comb-slit aur torch se 50°, 60° aur 70° pe kiran daali; har baar normal kheench kar protractor se naapa. Har kon pe 2 baar naap kar ausat (average) liya.
4. Result: 50° → 50°, 60° → 59°, 70° → 70° (lagbhag). 1° ka farq naapne ki galti hai.
5. Conclusion: kon chhota ho ya bada, i = r hi rehta hai. Anumaan galat nikla, aur yahi science hai: anumaan ko test karo, saboot maano. 🔬
🌍 Real Life Mein
- 🔭 Periscope: pandubbi (submarine) aur bunker mein do plane mirror 45° pe tirchhe lage hote hain. Har mirror pe i = r = 45° hone se roshni 90° mud jaati hai, aur neeche baithe log upar ka drishya dekh lete hain.
- 🌀 Kaleidoscope: andar tirchhe lage mirror baar-baar reflection karke rang-birange sundar pattern banate hain.
- 💇 Naai ki dukaan: aage-peeche lage do mirror se sir ka peechhla hissa dikh jaata hai; har mirror niyam ke hisaab se roshni lautata hai.
- 🚲 Cycle ke reflector: peeche lage laal reflector mein chhote-chhote kone wale tukde hote hain jo gaadi ki headlight ki roshni ko wapas usi taraf bhej dete hain, taaki raat mein cycle dikhe.
- 🛰️ Chaand pe laser: scientists ne Chaand pe khaas reflector rakhe hain. Dharti se bheji laser roshni wahan se lautti hai, aur aane-jaane ke samay se Chaand ki doori naapi jaati hai.
🔁 Recap: Poora Post Ek Nazar Mein
- Roshni ko ray (teer wali seedhi line) se dikhate hain.
- Incident ray mirror pe girti hai, reflected ray lautti hai; girne ka point = point of incidence (O).
- Normal = O pe mirror se 90° ki madadgaar line.
- i = incident ray aur normal ka kon; r = reflected ray aur normal ka kon.
- Niyam 1: i = r. Niyam 2: incident ray, normal, reflected ray ek hi samtal mein.
- Normal ke saath aayi kiran: i = r = 0°, usi raaste se wapas.
- Niyam sab mirrors (plane, concave, convex) pe chalte hain.
✍️ Fill in the Blanks
- Mirror pe girne wali kiran ko ______ ray kehte hain.
- Point of incidence pe mirror se 90° pe kheenchi line ______ kehlaati hai.
- Incident ray aur normal ke beech ke kon ko ______ kehte hain.
- Reflection ke pehle niyam ke anusaar angle of incidence aur angle of reflection ______ hote hain.
- Incident ray, normal aur reflected ray ek hi ______ mein hote hain.
- Kiran normal ke saath gire toh angle of reflection ______ hota hai.
Answers
1) incident · 2) normal · 3) angle of incidence (i) · 4) barabar (equal) · 5) samtal (plane) · 6) 0°
✅ True / False
- Normal mirror ki sateh ke parallel kheencha jaata hai.
- Angle of incidence hamesha angle of reflection ke barabar hota hai.
- Agar kiran mirror ki sateh se 20° ka kon banaye, toh angle of incidence 20° hai.
- Reflection ke niyam concave aur convex mirror pe bhi laagu hote hain.
- Incident ray, normal aur reflected ray ek hi samtal mein hote hain.
- Kiran normal ke saath gire toh reflection nahi hota.
Answers (kyun ke saath)
1) False: normal mirror pe 90° (lamb) hota hai.
2) True: yahi reflection ka pehla niyam hai.
3) False: i normal se naapte hain, toh i = 90° − 20° = 70°.
4) True: har mirror pe har kiran i = r maanti hai.
5) True: yahi doosra niyam hai.
6) False: reflection hota hai; i = r = 0° aur kiran usi raaste se laut aati hai.
🔘 MCQs
Q1. Normal kya hai? (a) Mirror ke parallel line (b) Point of incidence pe mirror se 90° ki line (c) Reflected ray (d) Incident ray
✅ (b): normal mirror pe lamb (90°) madadgaar line hai.
(a) parallel line mirror ki sateh jaisi hogi, normal nahi.
(c) reflected ray lautne wali kiran hai, normal nahi.
(d) incident ray girne wali kiran hai.
Q2. Angle of incidence 50° hai. Angle of reflection kitna hoga? (a) 40° (b) 50° (c) 90° (d) 100°
✅ (b): i = r, toh 50°.
(a) 40° toh reflected ray ka sateh se kon hai.
(c) 90° normal aur mirror ke beech ka kon hai.
(d) 100° dono kirnon ke beech ka kon (i + r) hai.
Q3. Angle of incidence kin do ke beech naapa jaata hai? (a) Incident ray aur mirror (b) Incident ray aur normal (c) Incident ray aur reflected ray (d) Reflected ray aur normal
✅ (b): incident ray aur normal ke beech.
(a) mirror se naapna aam galti hai.
(c) yeh dono kirnon ka kul kon (2i) hai.
(d) yeh angle of reflection hai.
Q4. Kiran mirror ki sateh se 30° ka kon banati hai. Angle of reflection? (a) 30° (b) 60° (c) 90° (d) 120°
✅ (b): i = 90° − 30° = 60°, aur r = i = 60°.
(a) 30° sateh se kon hai, normal se nahi.
(c) 90° tab hota jab kiran sateh ke saath chalti; tab reflection hi nahi.
(d) 120° dono kirnon ke beech ka kon (60° + 60°) hai.
Q5. Kiran mirror pe normal ke saath girti hai. Kya hoga? (a) i = 90° (b) i = r = 0° aur kiran usi raaste se lautegi (c) Reflection nahi hoga (d) r = 90°
✅ (b): kiran aur normal ke beech kon 0° hai, isliye r bhi 0°.
(a) i = 90° tab hota jab kiran sateh ke saath-saath chalti.
(c) reflection hota hai, bas seedha peeche.
(d) r = i = 0°, 90° nahi.
Q6. Chart sheet modne wali activity se kaunsa niyam sabit hota hai? (a) i = r (b) Incident ray, normal aur reflected ray ek hi samtal mein (c) Mirror image ulta banata hai (d) Convex mirror zyada area dikhata hai
✅ (b): mudi sheet naya samtal hai, isliye kiran us pe nahi dikhti.
(a) i = r kon naapne wali activity se sabit hota hai.
(c) yeh image ki baat hai, is activity se nahi.
(d) yeh convex mirror ka upyog hai, is activity ka nateeja nahi.
Q7. Reflection ke niyam kis par laagu hote hain? (a) Sirf plane mirror (b) Sirf concave mirror (c) Sirf convex mirror (d) Sab tarah ke mirrors
✅ (d): plane, concave, convex, sab pe har kiran i = r maanti hai.
(a), (b), (c) mein “sirf” galat hai, kyunki niyam kisi ek mirror tak seemit nahi.
Q8. Incident aur reflected ray ke beech 80° ka kon hai. Angle of incidence? (a) 80° (b) 40° (c) 50° (d) 160°
✅ (b): i + r = 80° aur i = r, toh i = 40°.
(a) 80° dono ka jod hai, akela i nahi.
(c) 50° reflected/incident ray ka sateh se kon hai (90° − 40°).
(d) 160° toh doguna kar diya, galat.
🔗 Assertion–Reason
Options: (a) A aur R dono sahi, aur R, A ko samjhata hai · (b) A aur R dono sahi, par R, A ko nahi samjhata · (c) A sahi, R galat · (d) A galat, R sahi
1. A: Normal ke saath girne wali kiran usi raaste se wapas laut aati hai. R: Aisi kiran ke liye i = 0°, isliye r bhi 0° hota hai.
✅ (a): dono sahi, aur r = 0° hona hi wajah hai ki kiran usi raaste se lautti hai.
2. A: Khurdari deewar mein humara chehra nahi dikhta. R: Reflection ke niyam sirf plane mirror pe laagu hote hain.
✅ (c): A sahi hai. R galat hai, niyam har sateh ki har kiran pe chalte hain; chehra isliye nahi dikhta kyunki khurdari sateh kirnon ko har taraf bikher deti hai.
3. A: Kiran normal ke saath gire toh angle of incidence 90° hota hai. R: Normal mirror ki sateh se 90° ka kon banata hai.
✅ (d): A galat hai, us case mein i = 0° hota hai (kon normal se naapte hain). R sahi hai.
📝 Very Short Answer (1 mark)
1. Mirror se lautne wali kiran ko kya kehte hain?
Reflected ray.
2. Angle of reflection kin do ke beech hota hai?
Reflected ray aur normal ke beech.
3. i = 65° ho toh r kitna?
65°.
4. Normal aur mirror ke beech kitna kon hota hai?
90°.
5. Patli kiran banane ke liye activity mein kis cheez ka use kiya?
Kanghi (comb) jiske ek daant ki jagah chhod kar baaki kaale kaagaz se dhaki ho, yaani slit.
📝 Short Answer (2–3 marks)
1. Reflection ke dono niyam likho.
(1) Angle of incidence, angle of reflection ke barabar hota hai (i = r). (2) Incident ray, point of incidence pe normal, aur reflected ray, teeno ek hi samtal (plane) mein hote hain.
2. Kiran mirror ki sateh se 35° ka kon banati hai. i, r aur dono kirnon ke beech ka kon nikaalo.
i = 90° − 35° = 55°. r = i = 55°. Dono kirnon ke beech kon = i + r = 55° + 55° = 110°.
3. Normal ko dotted line se kyun banate hain aur us pe teer kyun nahi lagate?
Normal roshni ki asli kiran nahi hai; yeh sirf kon naapne ke liye kheenchi gayi kalpanik madadgaar line hai. Roshni us pe chalti nahi, isliye na teer, aur dotted line se use kirnon se alag dikhate hain.
4. Chart sheet ka bahar wala hissa modne pe reflected kiran kyun gayab ho jaati hai?
Reflected kiran usi samtal mein rehti hai jisme incident kiran aur normal hain (yaani mez pe rakhi sheet ka samtal). Moda hua hissa ek naya samtal ban jaata hai, isliye kiran us pe nahi padti. Sheet seedhi karte hi samtal ek ho jaata hai aur kiran dikhne lagti hai.
📝 Long Answer (5 marks)
1. Ek activity ka varnan karo jisse sabit ho ki angle of incidence = angle of reflection. Saaf diagram ke baare mein bhi batao.
Saamaan: plane mirror, torch, kanghi, kaala kaagaz, safed kaagaz, pencil, protractor.
Tareeka: (1) Kanghi ke ek daant ki jagah chhod kar baaki ko kaale kaagaz se dhak do, taaki patli kiran bane. (2) Safed kaagaz pe mirror seedha khada karo aur uski line kheencho. (3) Andhere mein torch se patli kiran mirror pe daalo. (4) Aane wali aur lautne wali kiran ke raaste pe bindu lagao, mirror hatao, aur teer ke saath dono kirnein kheencho; milne ka point O hai. (5) O pe mirror line se 90° ka normal kheencho. (6) Protractor se i (incident ray aur normal) aur r (reflected ray aur normal) naapo. (7) Alag-alag kon pe dohrao aur table banao.
Diagram: neeche ek seedhi line (mirror), O pe khadi dotted line (normal), ek taraf se aati teer wali incident ray, doosri taraf jaati reflected ray, aur normal ke dono taraf i aur r ke kon.
Nateeja: har baar i aur r lagbhag barabar aate hain, isliye i = r.
2. Reflection ke niyam samjhao. Batao ki yeh curved mirror pe kaise laagu hote hain, aur khurdari sateh pe image kyun nahi banta.
Niyam: (1) i = r. (2) Incident ray, normal aur reflected ray ek samtal mein. Dono kon normal se naapte hain.
Curved mirror: concave aur convex mirror ek gole ka hissa hain. Har point pe normal woh line hai jo us point ko gole ke kendra se jodti hai. Har kiran apne point pe i = r maanti hai. Lekin har point ka normal alag disha mein hai, isliye concave mein kirnein aapas mein milti hain aur convex mein phailti hain.
Khurdari sateh: yahan bhi har kiran i = r maanti hai, par sateh ke chhote tukde alag-alag taraf mude hain, toh unke normal alag hain. Lauti kirnein har taraf bikhar jaati hain (diffused reflection), isliye saaf image nahi banta. Chikni sateh pe sab normal ek jaise hote hain, toh kirnein ek tarah lautti hain (regular reflection) aur image banta hai.
📰 Case-Based Question
Priya ne school lab mein comb-slit aur torch se reflection ki activity ki. Uski table: i = 20° pe r = 21°, i = 35° pe r = 35°, i = 50° pe r = 49°. Uske dost Rohan ne pehli reading mein galti se kon mirror ki sateh se naap liya aur “i = 70°” likh diya. Aakhir mein teacher ne kiran ko seedha mirror pe giraaya, aur kiran wapas slit pe hi aa gayi.
Q1. Priya ki readings se kya nishkarsh nikalta hai? 1° ka farq kyun aaya?
i aur r lagbhag barabar hain, yaani i = r. 1° ka farq naapne ki chhoti galti (mota beam, protractor ki jagah) ki wajah se hai.
Q2. Rohan ne sateh se 70° naapa. Sahi angle of incidence kya tha?
i = 90° − 70° = 20°. Yaani wahi kiran thi jiska i = 20° tha. Kon hamesha normal se naapna chahiye.
Q3. Teacher ke experiment mein i aur r kitne the? Kiran slit pe wapas kyun aayi?
Kiran normal ke saath giri, toh i = 0° aur r = 0°. Isliye woh usi raaste se laut kar slit pe aa gayi.
Q4. i = 35° wali reading mein dono kirnon ke beech kitna kon tha?
i + r = 35° + 35° = 70°.
🚀 HOTS
1. Deewar bhi roshni lautati hai, toh hum deewar mein apna chehra kyun nahi dekh paate?
Deewar ki sateh khurdari hai. Har kiran i = r maanti hai, par alag-alag tukdon ke normal alag disha mein hain, toh lauti kirnein har taraf bikhar jaati hain. Aankh tak ek saath, ek tarah se kirnein nahi pahunchti, isliye image nahi banta.
2. Seedhi (horizontal) aati kiran ko bilkul upar (vertical) bhejna hai. Plane mirror ko kaise rakhoge?
Kiran ko 90° modna hai. Incident aur reflected ray ke beech 90° chahiye, toh i + r = 90°, yaani i = r = 45°. Mirror ko kiran ki disha se 45° pe tirchha rakho (periscope mein yahi hota hai).
3. Dhoop ka dhabba door wali deewar pe ho toh sheesha thoda sa ghumane pe bhi bahut door kyun bhaagta hai?
Mirror ghumane pe normal ghoomta hai, aur reflected kiran mirror ke ghumaav se doguni ghoomti hai. Deewar jitni door, kiran ka chhota sa mudna bhi wahan utni badi doori ban jaata hai, jaise lambi chhadi ka sira thoda ghumane pe bahut ghoomta hai.
🏆 Olympiad Corner
1. Incident ray ko na hilaate hue plane mirror ko 10° ghumaya gaya. Reflected ray kitna ghoomegi? (a) 5° (b) 10° (c) 20° (d) 40°
✅ (c): mirror 10° ghoomne se normal 10° ghoomta hai; i 10° badalta hai aur r bhi, isliye reflected ray 2 × 10° = 20° ghoomti hai. (a) aadha, (b) mirror jitna, (d) chaar guna, teeno galat.
2. Do plane mirror aapas mein 90° pe rakhe hain. Kiran pehle mirror pe i = 30° se girti hai. Doosre mirror pe angle of incidence kya hoga? (a) 30° (b) 60° (c) 45° (d) 90°
✅ (b): pehle mirror se kiran apne normal se 30° pe lautti hai. Doosre mirror ka normal pehle ke normal se 90° pe hai, toh doosre normal se kon 90° − 30° = 60°. (a) tab hota jab mirror parallel hote; (c) aur (d) ka koi aadhar nahi.
3. Periscope mein seedhi aati roshni 45° tirchhe mirror pe girti hai. Angle of incidence kya hai? (a) 0° (b) 22.5° (c) 45° (d) 90°
✅ (c): kiran sateh se 45° banati hai, toh normal se bhi 90° − 45° = 45°. Phir r = 45° aur kiran 90° mud jaati hai. (a) normal ke saath hone pe, (d) sateh ke saath chalne pe, (b) ka koi aadhar nahi.
Har option pe tap karo, turant pata chalega sahi ya galat.
Q1. Angle of reflection 35° hai. Reflected ray aur mirror ki surface ke beech kitna angle hai?
(a) 35°
❌ Galat: Normal wala angle hi de diya: 35. Surface se poocha hai: 90° − 35° = 55°.
(b) 55°
✅ Sahi! Angle of reflection normal se naapte hain. Normal aur surface ke beech 90°: 90° − 35° = 55°.
(c) 70°
❌ Galat: Dono rays ke beech wala angle de diya: 35 + 35 = 70. Surface se: 90° − 35° = 55°.
(d) 125°
❌ Galat: 90° mein add kar diya: 90 + 35 = 125. Surface se angle: 90° − 35° = 55°.
Q2. Angle of incidence 28° hai. Incident ray aur reflected ray ke beech kitna angle hai?
(a) 28°
❌ Galat: Sirf ek angle liya: 28. Dono rays normal ke do taraf hain: 28° + 28° = 56°.
(b) 62°
❌ Galat: Surface se angle nikala: 90° − 28° = 62. Poocha dono rays ke beech: 28° + 28° = 56°.
(c) 56°
✅ Sahi! Angle of reflection bhi 28°, normal ki doosri taraf. Dono rays ke beech: 28° + 28° = 56°.
(d) 152°
❌ Galat: Straight line se subtract kiya: 180° − 28° = 152. Dono rays ke beech: 28° + 28° = 56°.
Q3. Ek student ne incident ray ka angle mirror ki surface se naap ke 60° likha. Asli angle of incidence kitna hai?
(a) 60°
❌ Galat: Surface se naapa angle hi likh diya: 60. Normal se naapo: 90° − 60° = 30°.
(b) 120°
❌ Galat: Angle double kar diya: 2 × 60 = 120. Normal se: 90° − 60° = 30°.
(c) 150°
❌ Galat: 90° mein add kiya: 90 + 60 = 150. Normal se: 90° − 60° = 30°.
(d) 30°
✅ Sahi! Angle of incidence normal se naapte hain. Normal surface se 90° pe: 90° − 60° = 30°.
Q4. Ek ray plane mirror pe 40° angle of incidence pe girti hai. Reflected ray kahan jaayegi?
(a) Normal ki doosri taraf, normal se 40° pe
✅ Sahi! Pehla law: angle of incidence = angle of reflection = 40°, aur reflected ray normal ki doosri taraf jaati hai.
(b) Normal ki usi taraf, normal se 40° pe
❌ Galat: Same side maana. Reflected ray normal ki doosri taraf jaati hai.
(c) Usi raaste se wapas
❌ Galat: Normal wala special case lagaya. Usi raaste se tab lautti hai jab angle of incidence 0° ho.
(d) Paper ke plane se bahar, upar ki taraf
❌ Galat: Doosra law bhool gaye. Incident ray, normal aur reflected ray ek hi plane mein rehte hain.
Q5. Mirror ko thoda tilt (thoda tedha) kiya, par ray ab bhi mirror ke naye normal ke saath hi girti hai. Kya hoga?
(a) Reflection nahi hoga
❌ Galat: Normal pe reflection band maana. Reflection hota hai; bas ray seedhi peeche lautti hai.
(b) Ray usi raaste se wapas lautegi
✅ Sahi! Normal ke saath girne pe angle of incidence 0°, to angle of reflection bhi 0°: ray usi raaste se lautti hai.
(c) Ray mirror ke andar se nikal jaayegi
❌ Galat: Mirror ko transparent maana. Mirror light ko wapas bhejta hai.
(d) Angle of reflection 90° hoga
❌ Galat: Normal aur surface ka 90° angle of reflection maana. Normal se angle 0°, to reflection bhi 0°.
📚 Aur Padho (External Reference)
- NCERT Class 8 Science Curiosity, Chapter 10 (official PDF)
- DIKSHA: Sarkar ka free learning platform, NCERT ke videos ke liye
💖 CharuMam Motivation
Mirror ka niyam bada seedha hai: roshni jis kon se aati hai, usi kon se laut jaati hai. Na kam, na zyada. i = r.
Zindagi bhi kuch aisi hi hai, beta. Jaisi mehnat aap roz padhai mein lagaoge, waisa hi nateeja laut kar aayega. Jaisa vyavhaar aap doosron se karoge, waisa hi pyaar aur izzat aapko milegi. Aur jaise normal ek seedhi, pakki line hai jo har kon ko sahi naapti hai, waise hi aap bhi apni imaandaari ki seedhi line pakde rakhna. Baaki sab apne aap sahi kon pe aa jaayega. ✨
— CharuMam
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