Lens Kya Hai? (Paani ki Boond ka Magnifying Glass)
Ek chhoti si paani ki boond akshar bade kaise dikha deti hai? Aaj milte hain lens se: convex aur concave! 🔍💧
🧭 Is Post Mein
📖 Kahani: Dadaji ka Gol Sheesha
Sunday ki subah thi. Dadaji akhbaar padh rahe the, par chhote akshar unhe saaf nahi dikh rahe the. Unhone apni daraaz se ek gol sheesha (magnifying glass) nikaala. Golu ne use uthakar akhbaar ke upar rakha: “Arre waah! Akshar toh itne bade ho gaye, jaise chhoti cheentiyan haathi ban gayi hon!”
Priya ke paas magnifying glass nahi tha. Usne ek saaf plastic sheet akhbaar pe rakhi aur ungli se uspe paani ki ek gol boond tapka di. “Dekho, meri boond ke neeche bhi akshar bade dikh rahe hain!”
Rohan bhaiya hanse: “Tum dono ke paas lens hai! Dadaji ka gol sheesha beech mein mota hai, aur tumhari boond bhi beech mein ubhri hui hai. Aise lens ko convex lens kehte hain. Mirror roshni ko lautata hai, par lens roshni ko apne andar se guzarne deta hai aur raaste mein use thoda mod deta hai.”
Pichhle post mein humne dekha ki concave mirror roshni ko ikattha karta hai. Aaj dekhenge ki lens kya hota hai, kitne prakar ka hota hai, aur woh roshni ke saath kya karta hai. 🔍
🎯 Is Post Mein Kya Seekhenge
- Lens kya hota hai: ek transparent cheez jiski kam se kam ek sateh curved (mudi hui) ho
- Convex lens (beech mein mota) aur concave lens (beech mein patla) ki pehchaan
- Mirror aur lens mein farq: mirror roshni lautata hai, lens roshni ko guzarne deta hai
- Convex lens parallel kirnon ko converge karta hai, concave lens unhe diverge karta hai
- Convex lens se dhoop ikatthi karna, aur focus aur focal length ka seedha matlab
- Paani ki boond bhi ek chhota sa convex lens hai
🧱 Basics: Pehle Yeh Aana Chahiye
1. Transparent cheezein: jinke aar-paar roshni aasani se nikal jaaye aur saaf dikhe, jaise saaf kaanch, saaf plastic, saaf paani.
2. Roshni seedhi chalti hai: ek hi cheez (jaise sirf hawa) mein roshni seedhi line mein chalti hai. Hum roshni ke raaste ko kiran (ray) se dikhate hain.
3. Converge aur diverge: kirnon ka paas aana = converge, door phailna = diverge. Concave mirror converge karta hai, convex mirror diverge.
Yeh bhool gaye? Pichhla post dekh lo: Mirror se Light ka Converge aur Diverge.
💡 Concept: Lens Roshni ko Modta Hai
Step 1: Lens kya hai? Lens kaanch, plastic ya kisi aur transparent cheez ka aisa tukda hai jiski sateh curved hoti hai (aksar dono taraf). Roshni iske aar-paar nikal jaati hai.
Step 2: Mirror se farq. Mirror ke peeche chamakdaar parat hoti hai, isliye roshni laut aati hai (reflection). Lens transparent hai, isliye roshni uske andar se guzarti hai. Par guzarte waqt roshni ki disha badal jaati hai.
Step 3: Roshni kyun mudti hai? Roshni hawa mein tez chalti hai aur kaanch ya paani mein thodi dheemi. Jab woh tirchhi hokar hawa se kaanch mein jaati hai (ya kaanch se hawa mein nikalti hai), toh uski disha thodi mud jaati hai. Is mudne ko refraction kehte hain. Lens ki curved sateh is mudne ko ek khaas pattern mein badal deti hai.
Step 4: Convex lens. Yeh beech mein mota aur kinaaron pe patla hota hai. Isse guzarne wali parallel kirnein mud kar ek-doosre ke paas aati hain aur ek jagah milti hain. Isliye ise converging lens bhi kehte hain. Dadaji ka magnifying glass aur paani ki boond, dono convex lens hain.
Step 5: Concave lens. Yeh beech mein patla aur kinaaron pe mota hota hai. Isse guzarne wali parallel kirnein ek-doosre se door phail jaati hain. Isliye ise diverging lens kehte hain.
🔸 Concave lens = beech mein patla → kirnein diverge (phailti hain)
🔸 Yaad rakhne ki trick: concave mein “cave” yaani gufa: beech mein andar dabba hua. Convex iska ulta: beech mein bahar ubhra hua.
Step 6: Pehchaan kaise karein? (1) Chhoo kar: kinaare pakad kar dhyaan se ungli phiraao; beech mota laga toh convex, patla laga toh concave. (2) Akshar dekh kar: lens ko akshar ke paas rakho; akshar bade dikhe toh convex, chhote dikhe toh concave.
Step 7: Dhoop aur convex lens. Convex lens dhoop ki parallel kirnon ko ek chhote, bahut chamakdaar dot pe ikattha kar deta hai. Wahan garmi itni ho sakti hai ki kaagaz jal jaaye. Concave lens se aisa dot kabhi nahi banta, kyunki woh roshni phailata hai.
🖼️ Diagrams: Picture se Samjho
🔐 Deep Concept: Focus, Do Focus aur Mota-Patla Lens
Raaz 1: Principal axis aur optical centre. Lens ke bilkul beech se, uski sateh ke seedhe (perpendicular) guzarne wali line ko principal axis kehte hain. Lens ka bilkul beech wala point optical centre hai. Is point se guzarne wali kiran lagbhag bina mude seedhi nikal jaati hai.
Raaz 2: Focus aur focal length. Principal axis ke parallel kirnein convex lens se nikal kar jis point pe milti hain, use focus (F) kehte hain. Lens se F ki doori focal length hai. Dhoop ka sabse chhota, sabse tez dot isi point pe banta hai.
🔸 Patle lens ke liye dono taraf ki focal length barabar hoti hai.
Raaz 3: Zyada mudaav = paas focus. Jis lens ki sateh zyada mudi (zyada gol) ho, woh roshni ko zyada modta hai, isliye uska focus paas banta hai (focal length kam). Chhoti paani ki boond bahut zyada gol hoti hai, isliye woh bahut taakatwar chhota lens ban jaati hai.
Raaz 4: Garmi kitni guna? Maan lo lens 50 cm² dhoop pakadta hai aur use 0.5 cm² ke dot mein ikattha kar deta hai. Toh dot pe har cm² pe lagbhag 100 guna zyada roshni girti hai (50 ÷ 0.5 = 100; thodi roshni kaanch mein kho jaati hai, isliye asal mein thoda kam). Bada lens zyada dhoop pakadta hai, isliye zyada garmi deta hai.
Raaz 5: Focal length naapne ka aasan tareeka. Convex lens ko dhoop mein kaagaz ke upar pakdo, aur upar-neeche karke sabse chhota dot dhoondho. Us waqt lens se kaagaz ki doori lagbhag focal length hai.
✏️ Examples: Convex ya Concave?
Paas rakhne pe akshar bade dikhte hain aur yeh beech mein mota hai.
Boond beech mein ubhri hui hai, aur neeche ke akshar bade dikhte hain.
Beech mein patla + akshar chhote = concave.
Sirf converging (convex) lens hi roshni ko ek dot pe ikattha karta hai.
Sabse chhota dot focus pe banta hai, aur lens se focus ki doori hi focal length hai.
60 ÷ 0.6 = 100. Kuch roshni kho jaati hai, isliye asal mein thoda kam.
⚖️ Difference Table: Convex Lens vs Concave Lens
| Baat | Convex lens | Concave lens |
|---|---|---|
| Aakaar | Beech mein mota, kinaare patle | Beech mein patla, kinaare mote |
| Doosra naam | Converging lens | Diverging lens |
| Parallel kirnein | Paas aa kar focus pe milti hain | Phail jaati hain |
| Paas rakhe akshar | Bade dikhte hain | Chhote dikhte hain |
| Dhoop ka dot / kaagaz jalna | Haan (sambhal ke!) | Nahi |
| Focus | Asli (roshni sach mein milti hai) | Nakli (roshni sirf wahan se aati lagti hai) |
| Ghar ka udaharan | Magnifying glass, paani ki boond | Kuch chashmon ke lens |
Aur mirror se farq? Mirror roshni ko lautata hai (reflection), lens roshni ko aar-paar jaane deta hai aur modta hai (refraction).
🚫 Misconceptions: Log aisa sochte hain… lekin
❌ “Lens bhi mirror ki tarah roshni lautata hai.”
✅ Sach: lens transparent hai; roshni uske aar-paar jaati hai aur mudti hai. Lautane ka kaam mirror ka hai.
❌ “Magnifying glass cheez ko sach mein bada kar deta hai.”
✅ Sach: cheez utni hi rehti hai; lens se aane wali mudi kirnon ki wajah se woh humein badi dikhti hai.
❌ “Lens sirf kaanch ka hota hai.”
✅ Sach: koi bhi transparent cheez jiski sateh curved ho, lens ban sakti hai: plastic, paani ki boond, paani bhara gol glass bhi.
❌ “Concave lens bhi dhoop ka dot bana kar kaagaz jala sakta hai.”
✅ Sach: concave lens roshni ko phailata hai, ikattha nahi karta. Dot sirf convex lens banata hai.
❌ “Convex lens se cheez hamesha badi aur seedhi dikhti hai.”
✅ Sach: sirf jab cheez lens ke paas ho. Door ki cheez convex lens se ulti bhi dikh sakti hai. Yeh agle post mein vistaar se!
🔤 Key Terms (Exam mein English shabd hi likhna)
❓ FAQs
1. Lens aur mirror dono curved hote hain, toh asli farq kya hai?
Mirror roshni ko wapas lautata hai (reflection). Lens transparent hai, roshni uske aar-paar jaati hai aur mudti hai (refraction). Isliye mirror mein hum apni taraf ki cheezein dekhte hain, aur lens ke paar doosri taraf ki.
2. Paani ki boond lens kaise ban jaati hai?
Plastic sheet pe boond gol ho kar beech mein ubhar jaati hai, yaani uski upar wali sateh curved hai aur beech mota. Yahi convex lens ka aakaar hai, isliye neeche ke akshar bade dikhte hain.
3. Bina chhooe convex aur concave lens kaise pehchaanein?
Lens ko kitaab ke akshar ke paas rakho. Akshar bade dikhe toh convex, chhote dikhe toh concave. Dhoop mein dot bane toh bhi convex.
4. Lens ke do focus kyun hote hain?
Roshni lens mein kisi bhi taraf se aa sakti hai. Baayein se aaye toh daayein taraf focus pe milegi, daayein se aaye toh baayein taraf. Isliye har taraf ek focus hota hai.
5. Kya lens ko dhoop mein khula chhodna khatarnaak hai?
Haan. Convex lens (ya paani bhari gol bottle) dhoop ko kisi kaagaz, kapde ya sookhe patton pe ikattha karke aag laga sakta hai. Lens ko hamesha dhak kar ya chhaaon mein rakho.
🧪 Activity (NCERT): Lens ko Pehchaano aur Roshni ka Raasta Dekho
Saamaan: ek convex lens, ek concave lens, kitaab ya akhbaar, torch, kanghi (comb), kaala kaagaz, safed kaagaz, ek purana kaagaz ka tukda (dhoop wale bhaag ke liye).
Bhaag 1: Chhoo kar aur dekh kar pehchaano
Bhaag 2: Kai kirnein, do lens
Bhaag 3: Dhoop ka dot (sirf bade ki nigraani mein)
Observation: convex lens beech mein mota, akshar bade dikhaata, kirnon ko ek jagah milata, aur dhoop ka chamakta dot banata hai; der tak rakhne pe kaagaz se dhuaan bhi nikal sakta hai. Concave lens beech mein patla, akshar chhote dikhaata, kirnon ko phailata, aur koi dot nahi banata.
Nateeja: convex lens = converging lens; concave lens = diverging lens. Dot jis doori pe sabse chhota bana, woh lagbhag lens ki focal length hai.
Yeh activity NCERT ki Class 8 Science book Curiosity (Chapter 10) par aadharit hai; yahan humne ise apne shabdon mein, step-by-step samjhaya hai.
🏠 Mini Experiments (Ghar pe)
Isse kya pata chala? Boond convex lens hai. Chhoti, zyada gol boond zyada mudi hui hai, isliye woh akshar zyada bade dikhaati hai.
Isse kya pata chala? Paani bhara gol glass bhi ek tarah ka convex lens hai. Door rakha teer ulta dikhta hai, paas rakha seedha. Lens ka asar cheez ki doori pe nirbhar hai.
Kisi bade ke chashme ko akshar ke upar thoda door rakho. Akshar bade dikhe toh uska lens convex hai, chhote dikhe toh concave. Chashma saaf haathon se pakdo aur kaanch ko mat chhuo.
⚠️ Safety Notes
🧠 Think Like a Scientist
Sawaal: Priya ki paani ki boond mein, chhoti boond akshar zyada bade dikhaati hai ya badi boond?
Scientist ka tareeka dekho
1. Observe (dekho): dono boondein akshar bade dikhaati hain, par barabar nahi.
2. Guess (anumaan): Golu ka anumaan: badi boond zyada bada dikhayegi, kyunki usme zyada paani hai.
3. Test (jaanch): ek hi akhbaar ke ek hi akshar pe teen alag size ki boondein (chhoti, beech wali, badi) rakhi aur har ek ke neeche akshar ki chaudai scale se andaaz se naapi. Teen baar dohraaya.
4. Result: sabse chhoti, sabse gol boond ne akshar sabse bade dikhaaye; badi, chapti boond ne kam.
5. Conclusion: lens ka asar paani ki maatra se nahi, uske mudaav se hota hai. Zyada mudi sateh roshni ko zyada modti hai. Golu ka anumaan galat nikla, aur yahi science hai: test karke seekhna! 🔬
🌍 Real Life Mein
- 🔍 Magnifying glass: ghadi theek karne wale, sunaar aur buzurg chhoti cheezein dekhne ke liye convex lens use karte hain.
- 👓 Chashme: kisi ko paas ka saaf nahi dikhta, kisi ko door ka. Unke chashmon mein zaroorat ke hisaab se convex ya concave lens lagta hai.
- 📷 Camera aur mobile: inme chhote-chhote lens roshni ko modkar saaf photo banaate hain.
- 🔬 Microscope aur 🔭 telescope: kai lens milkar bahut chhoti ya bahut door ki cheezein dikhaate hain.
- 👁️ Humari aankh: aankh ke andar bhi ek convex lens hai jo roshni ko modkar aankh ke peeche ki parat pe saaf chitra banata hai.
Lens se image kaisi banti hai aur yeh sab cheezein kaise kaam karti hain, yeh agle post mein vistaar se dekhenge.
🔁 Recap: Poora Post Ek Nazar Mein
- Lens = curved sateh wala transparent tukda; roshni uske aar-paar jaati hai aur mudti hai (refraction).
- Mirror roshni lautata hai, lens roshni guzarne deta hai.
- Convex lens: beech mein mota → parallel kirnon ko converge karta hai → dhoop ka dot banata hai.
- Concave lens: beech mein patla → parallel kirnon ko diverge karta hai → koi dot nahi.
- Pehchaan: chhoo kar (mota/patla) ya akshar dekh kar (bade = convex, chhote = concave).
- Parallel kirnein jahan milti hain = focus; lens se uski doori = focal length; lens ke do focus hote hain.
- Paani ki boond, paani bhara gol glass bhi lens ki tarah kaam karte hain.
- Lens se Suraj kabhi mat dekho, aur lens ko dhoop mein khula mat chhodo.
✍️ Fill in the Blanks
- Jo lens beech mein mota aur kinaaron pe patla ho, use ______ lens kehte hain.
- Concave lens parallel kirnon ko ______ karta hai.
- Lens se guzarte waqt roshni ka mudna ______ kehlaata hai.
- Convex lens ko ______ lens bhi kehte hain.
- Lens se focus ki doori ko ______ kehte hain.
- Plastic sheet pe rakhi gol paani ki boond ek ______ lens ki tarah kaam karti hai.
Answers
1) convex · 2) diverge · 3) refraction · 4) converging · 5) focal length · 6) convex
✅ True / False
- Lens roshni ko mirror ki tarah wapas lautata hai.
- Concave lens beech mein patla hota hai.
- Concave lens dhoop ka chamakta dot bana sakta hai.
- Convex lens ke paas rakhe akshar bade dikhte hain.
- Lens ke do focus hote hain, har taraf ek.
- Lens sirf kaanch se hi ban sakta hai.
Answers (kyun ke saath)
1) False: lens transparent hai, roshni aar-paar jaati hai; lautana mirror ka kaam hai.
2) True: concave lens beech mein patla, kinaaron pe mota hota hai.
3) False: concave lens roshni phailata hai, ikattha nahi karta.
4) True: isiliye magnifying glass convex lens hota hai.
5) True: roshni dono taraf se aa sakti hai, isliye har taraf ek focus.
6) False: plastic, paani ki boond jaisi koi bhi transparent curved cheez lens ban sakti hai.
🔘 MCQs
Q1. Lens kis tarah ki cheez se banta hai? (a) Opaque (b) Transparent (c) Chamakdaar dhaatu (d) Kaala plastic
✅ (b): roshni ko aar-paar jaana hai, isliye lens transparent hota hai.
(a) opaque cheez roshni rok degi.
(c) chamakdaar dhaatu roshni lautayegi, yeh mirror jaisa hai.
(d) kaala plastic roshni sokh lega.
Q2. Beech mein mota aur kinaaron pe patla lens kaunsa hai? (a) Concave lens (b) Plane glass (c) Convex lens (d) Convex mirror
✅ (c): convex lens beech mein mota hota hai.
(a) concave lens beech mein patla hota hai.
(b) plane glass har jagah barabar mota hota hai.
(d) convex mirror lens nahi, mirror hai; woh roshni lautata hai.
Q3. Parallel kirnein concave lens se guzarne ke baad kya karti hain? (a) Ek point pe milti hain (b) Phail jaati hain (c) Wapas laut aati hain (d) Gayab ho jaati hain
✅ (b): concave lens diverging lens hai.
(a) ek point pe milna convex lens ka kaam hai.
(c) lautna reflection hai, mirror mein hota hai.
(d) transparent lens roshni gayab nahi karta.
Q4. Dhoop mein kaagaz pe chamakta dot banane ke liye kya chahiye? (a) Concave lens (b) Plane mirror (c) Convex lens (d) Kaanch ki sapaat plate
✅ (c): convex lens dhoop ki kirnon ko ek jagah ikattha karta hai.
(a) concave lens roshni phailata hai.
(b) plane mirror lauti kirnon ko parallel hi rakhta hai, dot nahi banata.
(d) sapaat plate roshni ko ikattha nahi karti.
Q5. Lens se guzarte waqt roshni ke mudne ko kya kehte hain? (a) Reflection (b) Refraction (c) Dispersion (d) Absorption
✅ (b): refraction, kyunki roshni hawa se kaanch mein aur kaanch se hawa mein jaate waqt mudti hai.
(a) reflection matlab roshni ka lautna (mirror).
(c) dispersion matlab roshni ka alag-alag rangon mein bant jaana.
(d) absorption matlab roshni ka sokh liya jaana.
Q6. Ek lens se akshar chhote dikhte hain. Yeh kaunsa lens hai? (a) Convex (b) Concave (c) Magnifying glass (d) Paani ki boond
✅ (b): concave lens se cheezein chhoti dikhti hain.
(a) convex lens paas ke akshar bade dikhaata hai.
(c) magnifying glass convex hota hai, bada dikhaata hai.
(d) paani ki boond bhi convex jaisi hai, bada dikhaati hai.
Q7. Convex lens 15 cm upar rakhne pe dhoop ka sabse chhota dot bana. Uski focal length lagbhag kitni? (a) 7.5 cm (b) 15 cm (c) 30 cm (d) 45 cm
✅ (b): sabse chhota dot focus pe banta hai, toh focal length ≈ 15 cm.
(a) 7.5 cm aadhi doori hai, galat.
(c) 30 cm doguni doori hai, galat.
(d) 45 cm teen guna hai, galat.
Q8. Mirror aur lens mein mukhya farq kya hai? (a) Mirror roshni lautata hai, lens roshni guzarne deta hai (b) Dono roshni lautate hain (c) Lens roshni lautata hai, mirror guzarne deta hai (d) Dono roshni sokh lete hain
✅ (a): mirror = reflection, lens = refraction (aar-paar, mud kar).
(b) lens roshni nahi lautata.
(c) ulta likha hai.
(d) dono mein se koi bhi roshni ko mukhya roop se sokhta nahi.
🔗 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: Convex lens se dhoop kaagaz jala sakti hai. R: Convex lens parallel kirnon ko converge karke ek chhote dot pe ikattha karta hai.
✅ (a): dono sahi, aur roshni ka chhote dot pe ikattha hona hi garmi aur jalne ki wajah hai.
2. A: Concave lens ko diverging lens kehte hain. R: Concave lens beech mein mota hota hai.
✅ (c): A sahi hai, concave lens kirnein phailata hai. R galat hai: concave lens beech mein patla hota hai.
3. A: Lens roshni ko wapas lautata hai. R: Lens ek transparent cheez se banta hai.
✅ (d): A galat hai, lens roshni ko aar-paar jaane deta hai. R sahi hai, lens transparent hota hai.
📝 Very Short Answer (1 mark)
1. Lens kise kehte hain?
Curved sateh wala transparent tukda jo roshni ko aar-paar jaane deta hai aur modta hai.
2. Kaunsa lens parallel kirnon ko converge karta hai?
Convex lens.
3. Concave lens ka doosra naam kya hai?
Diverging lens.
4. Lens ke kitne focus hote hain?
Do (har taraf ek).
5. Magnifying glass mein kaunsa lens hota hai?
Convex lens.
📝 Short Answer (2–3 marks)
1. Convex aur concave lens mein aakaar ke hisaab se do farq likho.
Convex lens beech mein mota aur kinaaron pe patla hota hai; concave lens beech mein patla aur kinaaron pe mota. Convex ki sateh bahar ubhri hoti hai, concave ki andar dabi.
2. Bina kisi aur saamaan ke, sirf kitaab ki madad se convex aur concave lens kaise pehchanoge?
Lens ko kitaab ke akshar ke thoda upar rakh kar dekho. Akshar bade dikhe toh convex lens, chhote dikhe toh concave lens. Saath hi chhoone pe beech mota lage toh convex, patla lage toh concave.
3. Mirror aur lens mein kya farq hai? Ek-ek udaharan do.
Mirror roshni ko lautata hai (reflection), jaise ghar ka shisha ya gaadi ka side mirror. Lens roshni ko aar-paar jaane deta hai aur modta hai (refraction), jaise magnifying glass ya chashme ka lens.
4. Paani ki boond akshar bade kyun dikhaati hai, aur chhoti boond zyada kyun?
Boond beech mein ubhri hoti hai, yaani convex lens jaisi; isliye paas ke akshar bade dikhte hain. Chhoti boond zyada gol (zyada mudi) hoti hai, woh roshni ko zyada modti hai, isliye zyada bada dikhaati hai.
📝 Long Answer (5 marks)
1. Torch aur kanghi wali activity se dikhao ki convex aur concave lens parallel kirnon ke saath kya karte hain. Chitra ka varnan bhi karo.
Saamaan: torch, kanghi (ya jhiriyon wala kaala kaagaz), safed kaagaz, convex lens, concave lens.
Tareeka: kanghi se torch ki roshni ko kai patli parallel kirnon mein banaya aur safed kaagaz pe chalne diya. Raaste mein pehle convex lens, phir concave lens khada kiya, aur lens ke baad kirnon ka raasta pencil se banaya.
Observation: convex lens ke baad kirnein paas aa kar ek point pe mili; concave lens ke baad kirnein phail gayi.
Chitra: do chitra; har ek mein baayin taraf se parallel kirnein lens tak aati hain. Convex wale mein lens ke baad kirnein principal axis pe ek point F pe milti hain. Concave wale mein kirnein phailti hain aur unhe peeche badhane pe woh lens ke saamne ke ek point F se aati lagti hain.
Nateeja: convex lens = converging lens, concave lens = diverging lens.
2. Convex lens se focal length kaise naapoge? Is prayog mein kaun si saavdhaniyan zaroori hain?
Tareeka: dhoop mein convex lens ko ek purane kaagaz ke upar Suraj ki taraf karke pakdo. Lens ko dheere-dheere upar-neeche karo jab tak kaagaz pe sabse chhota, sabse chamakdaar dot na ban jaaye. Ab lens se kaagaz ki doori scale se naapo; yahi lagbhag focal length hai. Do-teen baar naap kar ausat lo.
Kyun: parallel dhoop ki kirnein focus pe milti hain, aur lens se focus ki doori hi focal length hai.
Saavdhaniyan: (1) lens se Suraj ko kabhi mat dekho. (2) prayog bade ki nigraani mein karo. (3) dot ko twacha, kapde ya sookhe patton pe mat daalo; paas mein paani rakho. (4) kaagaz aur lens ko sthir pakdo, aur doori lens ke beech se naapo.
📰 Case-Based Question
Priya ki teacher ne class ko teen transparent tukde diye: P, Q aur R. P ke paar akshar bade dikhe, Q ke paar chhote, aur R ke paar bilkul utne hi. Chhoone pe P beech mein mota laga aur Q beech mein patla. Dhoop mein P ko kaagaz se 10 cm upar rakhne pe sabse chhota chamakta dot bana.
Q1. P, Q aur R kya hain?
P = convex lens, Q = concave lens, R = sapaat kaanch (plane glass, lens nahi).
Q2. P ki focal length lagbhag kitni hai?
Lagbhag 10 cm, kyunki sabse chhota dot focus pe banta hai.
Q3. Q ko dhoop mein rakhne pe kya hoga?
Koi chamakta dot nahi banega, kyunki concave lens roshni ko phailata hai; kaagaz pe sirf halki roshni phailegi.
Q4. P ke saath kaam karte waqt ek zaroori saavdhaani batao.
P se Suraj ko kabhi mat dekho, aur dot ko twacha ya kapde pe mat daalo; prayog bade ki nigraani mein karo.
🚀 HOTS
1. Jungle mein pada saaf paani bhara plastic bottle kabhi-kabhi aag ka kaaran kyun ban sakta hai?
Paani bhari gol bottle convex lens jaisa kaam karti hai. Woh dhoop ko sookhe patton pe ek chhoti jagah ikattha kar sakti hai, jisse garmi badh kar aag lag sakti hai. Isliye kachra, khaas kar bottle aur kaanch, jungle mein nahi phenkna chahiye.
2. Do convex lens hain: ek zyada mota (zyada gol) aur ek patla (kam gol). Dhoop ka dot kiska paas banega aur kyun?
Zyada mote, zyada gol lens ka. Zyada mudi sateh roshni ko zyada modti hai, isliye kirnein jaldi milti hain aur focal length kam hoti hai.
3. Andhere kamre mein ek lens ke liye bina dhoop ke kaise pata karoge ki woh convex hai ya concave?
Torch aur kanghi se parallel kirnein banao aur lens ke paar bhejo. Kirnein ek jagah milein toh convex, phail jaayein toh concave. Ya lens se kitaab ke akshar dekho: bade = convex, chhote = concave.
🏆 Olympiad Corner
1. Ek lens 80 cm² dhoop ko 0.4 cm² ke dot pe ikattha karta hai. Roshni adhik se adhik kitni guna ghani hogi? (a) 20 (b) 32 (c) 200 (d) 320
✅ (c): 80 ÷ 0.4 = 200 guna (asal mein thodi roshni kaanch mein kho jaati hai, isliye thoda kam).
2. Inme se kaun lens ki tarah kaam NAHI kar sakta? (a) Paani ki gol boond (b) Paani bhara gol glass (c) Chamakdaar steel ki katori (d) Kaanch ki gol goli (marble)
✅ (c): steel ki katori opaque aur chamakdaar hai; woh roshni lautati hai (mirror jaisa), aar-paar nahi jaane deti. Baaki teeno transparent aur curved hain.
3. Ek convex lens ke ek taraf dhoop ka dot 20 cm pe banta hai. Lens ko palat kar (doosri sateh Suraj ki taraf) rakhne pe dot kitni door banega?
Lagbhag 20 cm hi. Patle lens ke dono taraf ki focal length barabar hoti hai, isliye palatne se focus ki doori nahi badalti.
Har option pe tap karo, turant pata chalega sahi ya galat.
Q1. Ek lens ko touch karke dekha: beech mein patla, edges pe mota. Yeh kaunsa lens hai, aur parallel rays ka kya karega?
(a) Convex lens: rays ko converge karega
❌ Galat: Mota-patla reverse kiya. Beech mein mota = convex; yahan beech patla hai: concave.
(b) Concave lens: rays ko converge karega
❌ Galat: Lens ka gun reverse kiya. Concave lens rays ko diverge karta hai.
(c) Convex lens: rays ko diverge karega
❌ Galat: Naam aur gun dono reverse kiye. Beech patla = concave, aur woh diverge karta hai.
(d) Concave lens: rays ko diverge karega
✅ Sahi! Beech mein patla = concave lens. Concave lens parallel rays ko diverge karta hai (spread).
Q2. Water ki ek drop ke neeche newspaper ke letters bade dikhe. Drop kaunse lens jaisi hai?
(a) Convex lens: beech mein moti
✅ Sahi! Drop beech mein ubhri, moti hai: convex lens. Isliye letters bade dikhe, magnifying glass ki tarah.
(b) Concave lens
❌ Galat: Concave ko bada dikhane wala maana. Concave lens letters chhote dikhata hai.
(c) Plane glass
❌ Galat: Drop ko flat maana. Drop ka beech ubhra hai; flat glass size nahi badalta.
(d) Mirror
❌ Galat: Drop ko mirror maana. Light drop ke through jaati hai: woh lens hai.
Q3. Lens light ki direction kyun badal deta hai (refraction)?
(a) Glass light ko wapas bhej deta hai
❌ Galat: Reflection ko refraction samjha. Refraction mein light glass ke through jaati hai, wapas nahi aati.
(b) Light air mein tez aur glass ya water mein thodi slow chalti hai
✅ Sahi! Air se glass mein jaate waqt speed badalti hai, isliye direction mudti hai: refraction. Curved surface isse pattern banati hai.
(c) Glass ke andar light ruk jaati hai
❌ Galat: Light ko rukta maana. Light thodi slow hoti hai, rukti nahi.
(d) Lens light ka colour badal deta hai
❌ Galat: Lens ko colour badalne wala maana. Refraction mein direction badalti hai, colour nahi.
Q4. Sunlight (dhoop) ka sabse chhota dot pehle lens se 10 cm pe aur doosre lens se 25 cm pe bana. Dono focal lengths mein kitna difference hai?
(a) 35 cm
❌ Galat: Difference ki jagah add kiya: 25 + 10 = 35. Difference = 25 − 10 = 15.
(b) 2.5 cm
❌ Galat: Divide kar diya: 25 ÷ 10 = 2.5. Difference = 25 − 10 = 15.
(c) 15 cm
✅ Sahi! Sabse chhota dot focus pe banta hai, to focal lengths 10 cm aur 25 cm. Difference = 25 − 10 = 15 cm.
(d) 25 cm
❌ Galat: Sirf doosre lens ki focal length de di (25). Difference = 25 − 10 = 15.
Q5. Concave lens se sunlight ka bright dot kyun nahi banta?
(a) Concave lens light ko rok deta hai
❌ Galat: Lens ko opaque maana. Lens transparent hai; bas rays ko spread karta hai.
(b) Concave lens converge karta hai, par dot bahut chhota hota hai
❌ Galat: Concave lens ko converge wala maana. Converge convex lens karta hai.
(c) Sunlight lens se reflect ho jaati hai
❌ Galat: Lens ko mirror maana. Lens mein light guzarti hai, wapas nahi aati.
(d) Concave lens rays ko diverge karta hai, woh ek point pe milti hi nahi
✅ Sahi! Concave lens parallel rays ko spread karta hai, isliye light ek chhoti jagah pe ikatthi nahi hoti.
📚 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
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