Lens se Images aur Upyog (Deewar pe Ulti Khidki!)
Magnifying glass se deewar pe poori khidki ulti kaise aa gayi? Aaj dekhte hain convex aur concave lens kaisi images banate hain, aur hum lens kahan-kahan use karte hain! 🔍🪟
🧭 Is Post Mein
📖 Kahani: Deewar pe Ulti Khidki
Dopahar thi. Verandah mein chhaaon thi, par saamne ki khidki dhoop se chamak rahi thi. Golu Dadaji ka magnifying glass le kar khel raha tha. Usne glass ko khidki aur deewar ke beech pakda, aur deewar pe chipke safed kaagaz ki taraf dheere-dheere aage-peeche kiya.
Achanak kaagaz pe khidki ki ek chhoti si, saaf, rangeen tasveer ban gayi! Priya chillaayi: “Golu, dekh! Khidki ke bahar ka ped bhi dikh raha hai… par yeh toh ulta hai! Ped ki jad upar aur patte neeche!”
Golu hairaan tha: “Kal toh isi glass se akhbaar ke akshar bade aur seedhe dikh rahe the. Aaj yeh chhoti aur ulti tasveer kaise?”
Rohan bhaiya muskuraaye: “Yahi toh lens ka kamaal hai! Convex lens kaisi image banayega, yeh is baat pe nirbhar hai ki cheez lens se kitni door hai. Door ki khidki → chhoti, ulti image jo kaagaz pe aa jaati hai. Paas ke akshar → badi, seedhi image jo sirf lens mein jhaank kar dikhti hai.”
Pichhle post mein humne seekha ki lens kya hai. Aaj dekhenge ki lens kaisi images banate hain, aur yeh chashme, camera, microscope jaisi cheezon mein kaise kaam aate hain. 🔍
🎯 Is Post Mein Kya Seekhenge
- Convex lens se door ki cheez ki image: ulti, chhoti, aur parde (screen) pe aa jaati hai
- Convex lens ke bahut paas rakhi cheez ki image: seedhi, badi, screen pe nahi aati (magnifying glass)
- Concave lens ki image: hamesha seedhi aur chhoti, screen pe kabhi nahi aati
- Real image aur virtual image ka farq
- Lens ke upyog: magnifying glass, chashme, camera, projector, microscope, telescope aur humari aankh
🧱 Basics: Pehle Yeh Aana Chahiye
1. Convex aur concave lens: convex beech mein mota hota hai aur roshni ko converge karta hai; concave beech mein patla hota hai aur roshni ko diverge karta hai.
2. Focus (F) aur focal length: parallel kirnein convex lens se nikal kar jis point pe milti hain, woh focus hai. Lens se focus ki doori focal length hai.
3. Image: kisi cheez se aayi roshni mud kar jahan milti hai (ya jahan se aati lagti hai), wahan humein us cheez ki image (pratibimb) dikhti hai. Mirror wale posts mein humne yahi dekha tha.
Yeh bhool gaye? Pichhla post dekh lo: Lens Kya Hai?
💡 Concept: Doori Badlo, Image Badlegi
Step 1: Door ki cheez + convex lens. Khidki, ped ya bulb jaisi door ki cheez se aane wali kirnein lens tak lagbhag parallel pahunchti hain. Convex lens unhe mod kar doosri taraf focus ke paas ek jagah mila deta hai. Wahan safed kaagaz (screen) rakho toh chhoti aur ulti image ban jaati hai.
Step 2: Image ulti kyun? Khidki ke upar wale hisse se aayi roshni lens se mud kar neeche jaati hai, aur neeche wale hisse ki roshni mud kar upar. Kirnein lens ke paas ek-doosre ko cross kar leti hain, isliye tasveer upar-neeche (aur daayein-baayein bhi) palat jaati hai.
Step 3: Real image. Jo image screen pe aa jaaye, use real image kehte hain, kyunki wahan roshni sach mein aa kar milti hai. Convex lens se screen pe bani image hamesha ulti hoti hai.
Step 4: Cheez ko paas laao. Jaise-jaise cheez lens ke paas aati hai (par focus se door rehti hai), screen wali image lens se door banti hai aur badi hoti jaati hai, par rehti ulti hi hai. Isi tarah projector chhoti film ki badi tasveer parde pe dikhata hai.
Step 5: Cheez bahut paas (focus ke andar). Ab lens ke paar kirnein milti hi nahi, bas thoda kam phailti hain. Lens mein jhaankne pe aankh ko lagta hai ki roshni ek badi, seedhi cheez se aa rahi hai. Yeh image screen pe nahi aati; ise virtual image kehte hain. Magnifying glass yahi karta hai.
Step 6: Concave lens. Concave lens roshni ko hamesha phailata hai, isliye kirnein kabhi milti nahi. Cheez kahin bhi ho, lens mein jhaankne pe image hamesha seedhi, chhoti aur virtual dikhti hai, aur screen pe kabhi nahi aati.
🔸 Convex + bahut paas ki cheez → seedhi, badi, virtual (screen pe nahi)
🔸 Concave + koi bhi doori → seedhi, chhoti, virtual (screen pe nahi)
🔸 Trick: screen pe aayi = real = ulti. Seedhi image lens se hamesha virtual hoti hai.
🖼️ Diagrams: Picture se Samjho
🔐 Deep Concept: Image ka Naksha Kaise Banaayein?
Raaz 1: Do aasan kirnein. Kisi bhi cheez ke sire se nikli bahut saari kirnon mein se sirf do kaafi hain: (1) jo axis ke parallel chale, woh convex lens se mud kar doosri taraf ke F se guzarti hai; (2) jo lens ke bilkul beech (optical centre) se guzre, woh seedhi nikal jaati hai. Dono jahan milein, wahan image ka sira hai. Upar ke diagrams aise hi banaaye gaye hain.
Raaz 2: Convex lens ka poora khel.
| Cheez kahan hai? | Image kahan? | Image kaisi? |
|---|---|---|
| Bahut door (jaise Suraj, door ka ped) | Lagbhag F pe | Bahut chhoti, ulti, real |
| 2F se aage | F aur 2F ke beech | Chhoti, ulti, real |
| Theek 2F pe | Doosri taraf 2F pe | Barabar size, ulti, real |
| F aur 2F ke beech | 2F se aage | Badi, ulti, real |
| F aur lens ke beech | Cheez wali taraf hi | Badi, seedhi, virtual |
Raaz 3: Kitni guna badi? Image ki lambai ÷ cheez ki lambai = magnification. Agar 1 cm ka akshar lens mein 3 cm ka dikhe, toh lens use 3 guna bada dikha raha hai.
Raaz 4: Humari aankh bhi camera hai. Aankh ka convex lens bahar ki duniya ki ulti, chhoti, real image aankh ke peeche ki parat (retina) pe banata hai. Phir dimaag us image ko seedha samajh leta hai. Camera mein bhi lens isi tarah sensor pe ulti image banata hai.
Raaz 5: Focal length ka shortcut. Bahut door ki cheez (jaise door ki khidki) ki saaf image convex lens lagbhag focus pe banata hai. Toh lens se screen ki doori ≈ focal length. Yeh dhoop wale tareeke se zyada surakshit hai!
✏️ Examples: Image Kaisi Banegi?
Door ki cheez + convex lens = screen pe aane wali ulti, chhoti image.
Cheez focus ke andar hai, isliye image badi aur seedhi; screen pe nahi aayegi.
Concave lens hamesha aisi hi image banata hai.
Bahut door ki cheez ki image lagbhag focus pe banti hai.
8 ÷ 2 = 4.
Concave lens ki image virtual hoti hai; roshni kabhi sach mein milti hi nahi.
⚖️ Difference Table: Real Image vs Virtual Image
| Baat | Real image | Virtual image |
|---|---|---|
| Roshni | Sach mein aa kar milti hai | Sirf wahan se aati lagti hai |
| Screen pe | Aa jaati hai | Nahi aati |
| Disha (lens/mirror ki) | Hamesha ulti | Hamesha seedhi |
| Lens ki kis taraf | Cheez ke ulti taraf | Cheez wali taraf |
| Kaun banata hai | Convex lens (cheez F se door ho) | Concave lens hamesha; convex lens jab cheez F ke andar ho; plane mirror |
| Udaharan | Projector ka parda, camera, aankh ka retina | Magnifying glass, shisha, concave lens |
🚫 Misconceptions: Log aisa sochte hain… lekin
❌ “Convex lens hamesha cheez ko bada dikhata hai.”
✅ Sach: sirf jab cheez lens ke paas (focus ke andar) ho. Door ki cheez ki image convex lens chhoti aur ulti banata hai.
❌ “Virtual image nakli hai, isliye dikhti hi nahi.”
✅ Sach: virtual image aankh se saaf dikhti hai (jaise shishe mein apna chehra); bas use screen pe nahi laaya ja sakta.
❌ “Concave lens se bhi kabhi-kabhi screen pe image aa jaati hai.”
✅ Sach: concave lens roshni hamesha phailata hai, isliye uski image kabhi screen pe nahi aati.
❌ “Hum duniya ulti dekhte hain kyunki aankh mein ulti image banti hai.”
✅ Sach: retina pe image ulti hi banti hai, par dimaag use apne aap seedha samajh leta hai.
❌ “Screen pe bani image sirf upar-neeche palatti hai.”
✅ Sach: woh daayein-baayein bhi palat jaati hai. Khidki ki baayin taraf ka ped image mein daayin taraf dikhega.
🔤 Key Terms (Exam mein English shabd hi likhna)
❓ FAQs
1. Ek hi magnifying glass kabhi badi seedhi, kabhi chhoti ulti image kyun banata hai?
Image cheez ki doori pe nirbhar hai. Cheez focus ke andar ho toh badi aur seedhi (virtual) image dikhti hai. Cheez focus se door ho toh image ulti (real) banti hai, aur bahut door ho toh chhoti bhi.
2. Real aur virtual image ko pehchaanne ka sabse aasan tareeka?
Screen (safed kaagaz) rakh kar dekho. Image kaagaz pe aa gayi toh real, nahi aayi par lens mein jhaankne pe dikhi toh virtual.
3. Concave lens kis kaam aata hai jab woh chhota hi dikhata hai?
Jin logon ko door ki cheezein dhundhli dikhti hain, unke chashmon mein concave lens lagta hai. Woh roshni ko thoda phaila kar aankh ke andar image ko sahi jagah le aata hai. Kuch darwaazon ke “peephole” mein bhi aise lens hote hain jo bahar ka bada hissa chhota karke dikhate hain.
4. Projector chhoti film ki itni badi tasveer kaise banata hai?
Projector mein film convex lens ke F aur 2F ke beech rakhi jaati hai. Aisi cheez ki image lens se door, badi aur ulti banti hai. Isliye film ko pehle se ulta lagaate hain, taaki parde pe tasveer seedhi dikhe.
5. Kya camera ke lens ki image bhi ulti hoti hai?
Haan. Camera ka convex lens sensor (ya purane camera mein film) pe ulti, chhoti, real image banata hai. Camera ka computer use seedha karke screen pe dikhata hai.
🧪 Activity (NCERT): Lens se Bani Images ko Pakdo
Saamaan: ek convex lens (magnifying glass), ek concave lens, safed kaagaz ya gatta (screen), scale, kitaab, aur ek kamra jiski khidki ke bahar ujaala ho.
Bhaag 1: Door ki cheez ki image screen pe
Bhaag 2: Paas ki cheez lens mein
Bhaag 3: Concave lens
Observation: convex lens ne khidki ki chhoti, ulti image kaagaz pe banayi (real image). Paas ke akshar lens mein bade aur seedhe dikhe, par kaagaz pe nahi aaye (virtual image). Concave lens se kaagaz pe koi image nahi bani; jhaankne pe har cheez chhoti aur seedhi dikhi.
Nateeja: convex lens cheez ki doori ke hisaab se real ya virtual image banata hai. Concave lens hamesha seedhi, chhoti, virtual image banata hai. Khidki ki image jis doori pe saaf bani, 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? Deewar pe tasveer badi aur ulti aati hai (real image). Seedhi tasveer chahiye toh mobile ko ulta rakho. Asli projector bhi aise hi kaam karta hai.
Isse kya pata chala? Paas se naam bada aur seedha dikhta hai; door se chhota aur ulta (upar-neeche aur daayein-baayein dono) dikhta hai.
Kisi bade ke chashme se khidki ki image kaagaz pe banane ki koshish karo. Image ban gayi toh lens convex hai; nahi bani aur cheezein chhoti dikhi toh concave. Chashma saaf haathon se, kinaaron se pakdo.
⚠️ Safety Notes
🧠 Think Like a Scientist
Sawaal: Golu ke lens se khidki ki image kahan banti hai: kya cheez paas laane pe screen ko bhi khiskana padega?
Scientist ka tareeka dekho
1. Observe (dekho): door ki khidki ki saaf image lens se lagbhag 15 cm pe bani.
2. Guess (anumaan): Priya ka anumaan: cheez kahin bhi ho, image hamesha 15 cm pe hi banegi.
3. Test (jaanch): kamre mein ek jalta bulb (ya torch) pehle 2 meter, phir 1 meter, phir 50 cm door rakha. Har baar screen ko khiska kar saaf image dhoondhi aur lens se screen ki doori naapi.
4. Result: bulb jitna paas aaya, saaf image lens se utni door bani aur utni badi hoti gayi. Image har baar ulti thi.
5. Conclusion: sirf bahut door ki cheez ki image lagbhag focus pe banti hai. Cheez paas aaye toh image door aur badi banti hai. Priya ka anumaan galat nikla, par test ne sach bata diya! 🔬
🌍 Real Life Mein
- 🔍 Magnifying glass: sunaar, ghadi-saaz, aur chhote akshar padhne wale buzurg; cheez focus ke andar rakh kar badi seedhi image dekhte hain.
- 👓 Chashme: jinhe paas ka saaf nahi dikhta unke chashme mein aksar convex lens, aur jinhe door ka saaf nahi dikhta unke chashme mein concave lens lagta hai. Chashma hamesha aankhon ke doctor ki salaah se banwao.
- 📷 Camera aur mobile: lens sensor pe chhoti, ulti, real image banata hai.
- 📽️ Projector: chhoti tasveer ki badi, real image parde pe.
- 🔬 Microscope: do convex lens milkar bahut chhoti cheezein, jaise pyaaz ki jhilli ke cells, badi dikhaate hain.
- 🔭 Telescope / doorbeen: lens door ke chaand-taaron aur pahadon ko saaf aur paas jaisa dikhaate hain.
- 👁️ Humari aankh: aankh ka convex lens retina pe ulti image banata hai, aur dimaag use seedha kar deta hai.
🔁 Recap: Poora Post Ek Nazar Mein
- Convex lens + door ki cheez → ulti, chhoti, real image (screen pe aati hai), lagbhag focus ke paas.
- Cheez paas aaye (par F se door) → image lens se door, badi, par phir bhi ulti aur real.
- Convex lens + cheez F ke andar → seedhi, badi, virtual image: magnifying glass.
- Concave lens → hamesha seedhi, chhoti, virtual image; screen pe kabhi nahi.
- Real image screen pe aati hai aur ulti hoti hai; virtual image screen pe nahi aati aur seedhi hoti hai.
- Magnification = image ki lambai ÷ cheez ki lambai.
- Upyog: magnifying glass, chashme, camera, projector, microscope, telescope, aur humari aankh.
- Lens se Suraj kabhi mat dekho.
✍️ Fill in the Blanks
- Jo image screen pe aa jaaye, use ______ image kehte hain.
- Concave lens ki image hamesha seedhi aur ______ hoti hai.
- Magnifying glass mein cheez lens aur ______ ke beech rakhi jaati hai.
- Humari aankh mein image ______ pe banti hai.
- Convex lens se door ki cheez ki image chhoti aur ______ banti hai.
- Image ki lambai ko cheez ki lambai se bhaag dene pe ______ milta hai.
Answers
1) real · 2) chhoti (diminished) · 3) focus (F) · 4) retina · 5) ulti (inverted) · 6) magnification
✅ True / False
- Virtual image ko screen pe laaya ja sakta hai.
- Concave lens ki image hamesha seedhi hoti hai.
- Convex lens hamesha cheez ko bada dikhata hai.
- Lens se screen pe bani image hamesha ulti hoti hai.
- Camera ka lens sensor pe seedhi image banata hai.
- Cheez convex lens ke 2F pe ho toh image cheez ke barabar size ki banti hai.
Answers (kyun ke saath)
1) False: virtual image mein roshni sach mein milti hi nahi, isliye screen pe kuch nahi aata.
2) True: concave lens hamesha seedhi, chhoti, virtual image banata hai.
3) False: door ki cheez ki image convex lens chhoti banata hai.
4) True: screen pe aane wali (real) image lens se hamesha ulti hoti hai.
5) False: sensor pe image ulti banti hai; camera use baad mein seedha dikhata hai.
6) True: tab image doosri taraf 2F pe, barabar size ki aur ulti banti hai.
🔘 MCQs
Q1. Convex lens se door ki khidki ki image screen pe kaisi banti hai? (a) Seedhi aur badi (b) Ulti aur chhoti (c) Seedhi aur chhoti (d) Image banti hi nahi
✅ (b): door ki cheez ki real image ulti aur chhoti hoti hai.
(a) seedhi badi image tab dikhti hai jab cheez F ke andar ho, aur woh screen pe nahi aati.
(c) seedhi chhoti image concave lens ki hoti hai.
(d) convex lens door ki cheez ki image zaroor banata hai.
Q2. Magnifying glass kaisi image dikhata hai? (a) Real, ulti, badi (b) Virtual, seedhi, badi (c) Real, seedhi, chhoti (d) Virtual, ulti, chhoti
✅ (b): cheez F ke andar hoti hai, isliye image virtual, seedhi aur badi.
(a) real ulti badi image projector jaisi hoti hai, magnifying glass mein jhaank kar nahi.
(c) lens ki real image seedhi nahi hoti.
(d) virtual image ulti nahi hoti.
Q3. Kaunsa lens kabhi bhi screen pe image nahi bana sakta? (a) Convex lens (b) Magnifying glass (c) Concave lens (d) Camera ka lens
✅ (c): concave lens roshni phailata hai, uski image hamesha virtual.
(a) convex lens door ki cheez ki image screen pe banata hai.
(b) magnifying glass bhi convex lens hai; khidki ki image bana sakta hai.
(d) camera ka lens sensor pe real image banata hai.
Q4. Real image ki pehchaan kya hai? (a) Woh hamesha seedhi hoti hai (b) Woh screen pe aa jaati hai (c) Woh sirf mirror banata hai (d) Woh hamesha badi hoti hai
✅ (b): real image mein roshni sach mein milti hai, isliye screen pe aa jaati hai.
(a) lens ki real image ulti hoti hai.
(c) convex lens bhi real image banata hai.
(d) real image chhoti, barabar ya badi, teeno ho sakti hai.
Q5. Projector mein kaunsa lens hota hai aur film kaise lagaate hain? (a) Concave, seedhi (b) Convex, ulti (c) Concave, ulti (d) Convex, seedhi
✅ (b): convex lens badi ulti real image banata hai, isliye film ulti lagaate hain taaki parde pe tasveer seedhi dikhe.
(a) concave lens screen pe image nahi banata.
(c) concave lens wahi galti.
(d) film seedhi lagaane pe parde pe tasveer ulti aayegi.
Q6. Humari aankh ke retina pe image kaisi banti hai? (a) Seedhi, virtual (b) Ulti, real (c) Seedhi, real (d) Ulti, virtual
✅ (b): aankh ka convex lens retina pe ulti, chhoti, real image banata hai; dimaag use seedha samajhta hai.
(a) retina ek parde ki tarah hai, us pe virtual image nahi ban sakti.
(c) lens ki real image seedhi nahi hoti.
(d) virtual image parde pe nahi banti.
Q7. 3 mm ka akshar magnifying glass mein 12 mm ka dikhta hai. Magnification? (a) 3 (b) 4 (c) 9 (d) 36
✅ (b): 12 ÷ 3 = 4 guna.
(a) 3 akshar ki lambai hai, magnification nahi.
(c) 12 − 3 = 9 ghataana galat tareeka hai.
(d) 12 × 3 = 36 guna karna galat hai.
Q8. Door ke ped ki saaf image convex lens se 20 cm door bani. Lens ki focal length lagbhag? (a) 10 cm (b) 20 cm (c) 40 cm (d) Pata nahi chal sakta
✅ (b): bahut door ki cheez ki image lagbhag focus pe banti hai, toh f ≈ 20 cm.
(a) 10 cm aadhi doori hai.
(c) 40 cm doguni doori hai.
(d) door ki cheez se focal length ka andaaza aasani se lagta hai.
🔗 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: Concave lens ki image screen pe nahi aati. R: Concave lens se nikli kirnein phail jaati hain aur kabhi sach mein nahi milti.
✅ (a): dono sahi, aur kirnon ka kabhi na milna hi image ke virtual hone ki wajah hai.
2. A: Magnifying glass mein akshar ulte dikhte hain. R: Magnifying glass ek convex lens hai.
✅ (d): A galat hai: paas rakhe akshar magnifying glass mein seedhe aur bade dikhte hain. R sahi hai.
3. A: Camera ke sensor pe image ulti banti hai. R: Camera ke lens se bani image real hoti hai, aur lens ki real image ulti hoti hai.
✅ (a): dono sahi, aur real image ka ulta hona hi A ki wajah hai.
📝 Very Short Answer (1 mark)
1. Virtual image kise kehte hain?
Woh image jo screen pe nahi aati, sirf lens ya mirror mein jhaank kar dikhti hai.
2. Concave lens ki image kaisi hoti hai?
Hamesha seedhi, chhoti aur virtual.
3. Projector mein kaunsa lens hota hai?
Convex lens.
4. Aankh ke peeche ki parat jahan image banti hai, uska naam?
Retina.
5. Bahut chhoti cheezein dekhne ke yantra ka naam?
Microscope.
📝 Short Answer (2–3 marks)
1. Real aur virtual image mein teen farq likho.
(1) Real image screen pe aa jaati hai, virtual nahi. (2) Real image mein roshni sach mein milti hai, virtual mein sirf wahan se aati lagti hai. (3) Lens ki real image ulti hoti hai, virtual seedhi.
2. Ek hi convex lens se kabhi badi seedhi aur kabhi chhoti ulti image kyun dikhti hai?
Image cheez ki doori pe nirbhar hai. Cheez focus ke andar ho toh badi, seedhi, virtual image dikhti hai. Cheez door ho toh image ulti aur real banti hai, aur bahut door ho toh chhoti bhi.
3. Bina chhuye, sirf khidki aur kaagaz se kaise pata karoge ki lens convex hai ya concave?
Lens ko khidki aur kaagaz ke beech aage-peeche karo. Khidki ki ulti image kaagaz pe aa gayi toh lens convex hai. Kahin bhi image na bane aur lens mein jhaankne pe cheezein chhoti seedhi dikhein toh concave.
4. Lens ke chaar upyog likho.
Magnifying glass (chhoti cheezein bada dekhna), chashme (nazar theek karna), camera (photo lena), microscope (cells jaisi chhoti cheezein dekhna). Projector aur telescope bhi sahi hain.
📝 Long Answer (5 marks)
1. Ek activity se dikhao ki convex lens door ki cheez ki real image banata hai, aur concave lens nahi. Observation aur nateeja likho.
Saamaan: convex lens, concave lens, safed kaagaz (screen), scale, khidki wala kamra.
Tareeka: kamre ki light band ki. Convex lens ko chamakti khidki aur deewar pe lage kaagaz ke beech pakda aur aage-peeche kiya jab tak saaf image na bani. Lens se kaagaz ki doori naapi. Phir concave lens ke saath yahi kiya.
Observation: convex lens se khidki ki chhoti, ulti, saaf image kaagaz pe bani. Concave lens se koi image nahi bani, kaagaz pe sirf halki roshni phaili.
Kyun: convex lens door se aayi kirnon ko mod kar ek jagah milata hai, isliye real image banti hai. Concave lens kirnon ko phailata hai, woh kabhi milti nahi.
Nateeja: convex lens real image bana sakta hai; concave lens ki image hamesha virtual hoti hai. Saaf image ki doori lagbhag convex lens ki focal length hai.
2. Convex lens ke saamne cheez ko bahut door se dheere-dheere paas laaya jaaye toh image kaise badalti hai? Har sthiti ka ek upyog bhi batao.
(1) Bahut door: image lagbhag F pe, bahut chhoti, ulti, real. Upyog: camera, aankh.
(2) 2F se aage: F aur 2F ke beech, chhoti, ulti, real. Upyog: camera.
(3) 2F pe: doosri taraf 2F pe, barabar size, ulti, real. Upyog: photocopy jaisi barabar size ki copy.
(4) F aur 2F ke beech: 2F se aage, badi, ulti, real. Upyog: projector.
(5) F aur lens ke beech: cheez wali taraf, badi, seedhi, virtual. Upyog: magnifying glass.
Saar: cheez jitni paas aati hai (F tak), real image utni door aur badi hoti jaati hai; F ke andar aate hi image virtual aur seedhi ho jaati hai.
📰 Case-Based Question
Science fair ke liye Rohan ne joote ke dibbe mein ek convex lens lagaya jiski focal length 6 cm thi. Usne mobile ko lens se 8 cm door rakha. Lens se 24 cm door deewar pe mobile ki photo ki saaf tasveer bani. Mobile pe photo 5 cm oonchi thi.
Q1. Deewar pe bani tasveer kaisi hogi?
Badi, ulti aur real, kyunki mobile F (6 cm) aur 2F (12 cm) ke beech hai.
Q2. Logon ko seedhi tasveer dikhane ke liye Rohan kya kare?
Mobile ko dibbe mein ulta (upar-neeche) rakh de; lens tasveer ko phir se palat kar deewar pe seedha dikha dega.
Q3. Agar mobile lens se sirf 4 cm door ho, toh deewar pe kya dikhega?
Deewar pe koi saaf tasveer nahi aayegi, kyunki ab mobile F ke andar hai. Image virtual, seedhi aur badi hogi, jo sirf lens mein jhaankne pe dikhegi.
Q4. Deewar pe tasveer 15 cm oonchi bani. Magnification kitna hai?
15 ÷ 5 = 3 guna. (Yeh image ki doori ÷ cheez ki doori = 24 ÷ 8 = 3 se bhi milta hai.)
🚀 HOTS
1. Priya kehti hai, “Mere lens se cheezein seedhi aur chhoti dikhti hain, toh yeh convex hai.” Kya woh sahi hai?
Nahi. Convex lens jab bhi seedhi image dikhata hai, woh hamesha badi hoti hai. Seedhi aur chhoti image sirf concave lens deta hai. Isliye Priya ka lens concave hai.
2. Convex lens ka aadha hissa kaale kaagaz se dhak diya. Kya screen pe khidki ki aadhi image banegi?
Nahi, poori image banegi, par dhundhli/kam chamakdaar. Khidki ke har point se roshni lens ke har hisse pe girti hai, toh khula aadha hissa bhi poori image bana deta hai; bas roshni aadhi aati hai.
3. Aankh mein image ulti banti hai, phir bhi naya paida hua bachcha duniya ko kaise seedha dekhna seekh leta hai?
Dimaag aankh se aayi image ko samajhna seekhta hai. Haath se chhoo kar aur chal-phir kar woh jodta hai ki “upar” kya hai aur “neeche” kya. Is tarah dimaag ulti image ko apne aap seedha maan leta hai.
🏆 Olympiad Corner
1. 10 cm focal length wale convex lens se 20 cm door ek cheez rakhi. Image kahan aur kaisi banegi? (a) 10 cm, chhoti (b) 20 cm, barabar, ulti (c) 40 cm, badi (d) Cheez wali taraf, seedhi
✅ (b): 20 cm = 2F, toh image doosri taraf 2F (20 cm) pe, barabar size ki, ulti aur real banti hai.
2. Inme se kaun screen pe image bana sakta hai? (a) Concave lens (b) Plane mirror (c) Convex lens, cheez F se door (d) Convex lens, cheez F ke andar
✅ (c): sirf yahi sthiti real image deti hai. Baaki teeno virtual image banate hain.
3. Ek lens se 4 cm ki cheez ki image 1 cm ki, seedhi dikhti hai. Lens kaunsa hai aur magnification kitna?
Seedhi aur chhoti image = concave lens. Magnification = 1 ÷ 4 = 0.25 (yaani image cheez ki ek-chauthaai).
Har option pe tap karo, turant pata chalega sahi ya galat.
Q1. Ek lens ne candle ki image white paper (screen) pe bana di. Image ke baare mein kya pakka hai?
(a) Real aur inverted; lens convex hai
✅ Sahi! Screen pe aayi image real hoti hai, aur lens se bani real image hamesha inverted. Screen image sirf convex lens deta hai.
(b) Virtual aur erect
❌ Galat: Screen wali image ko virtual maana. Screen pe aaye = real, aur real = inverted.
(c) Real aur erect
❌ Galat: Real ko erect maana. Lens se screen pe aayi image hamesha inverted hoti hai.
(d) Lens concave hai
❌ Galat: Concave lens se screen image maani. Concave lens ki image kabhi screen pe nahi aati.
Q2. Magnifying glass se letters bade aur erect dikhe. Letters lens se kahan hain?
(a) Focus se bahut door
❌ Galat: Door wali position li. Door se convex lens inverted image deta hai.
(b) Lens ke bahut paas (focus ke andar)
✅ Sahi! Focus ke andar rakhi cheez ka convex lens mein image erect, enlarged aur virtual hota hai.
(c) Lens ki doosri taraf, screen pe
❌ Galat: Image ko screen pe maana. Badi-erect image virtual hai, screen pe nahi aati.
(d) Kahin bhi: convex lens hamesha bada aur erect dikhata hai
❌ Galat: Ek position ko hamesha maana. Door rakhne pe image inverted ho jaati hai.
Q3. 5 mm ka insect magnifying glass mein 20 mm ka dikha. Image insect se kitne times badi hai (magnification)?
(a) 15 times
❌ Galat: Subtract kar diya: 20 − 5 = 15. Kitne times: 20 ÷ 5 = 4.
(b) 100 times
❌ Galat: Multiply kar diya: 20 × 5 = 100. Sahi: 20 ÷ 5 = 4.
(c) 4 times
✅ Sahi! Magnification = image ka size ÷ object ka size = 20 ÷ 5 = 4 times.
(d) 0.25 times
❌ Galat: Divide reverse kiya: 5 ÷ 20 = 0.25. Sahi: 20 ÷ 5 = 4.
Q4. Concave lens (object paas), concave lens (object door), convex lens (object door), convex lens (object focus ke andar): kitne cases mein image screen pe aa sakti hai?
(a) 2
❌ Galat: Convex (focus ke andar) ko bhi gina: 2. Woh image virtual hai, screen pe nahi: 2 − 1 = 1.
(b) 3
❌ Galat: Dono concave bhi gine: 3. Concave lens kabhi screen image nahi deta: 3 − 2 = 1.
(c) 0
❌ Galat: Convex (door) bhi chhod diya: 0. Door ki cheez ki convex image screen pe aati hai: 0 + 1 = 1.
(d) 1
✅ Sahi! Screen pe sirf real image aati hai: convex lens + door ka object (1). Baaki teen virtual hain.
Q5. Convex lens ke front mein candle ko door se dheere-dheere paas laaye (par focus se door hi rakha). Screen wali image ka kya hoga?
(a) Badi hoti jaayegi, par inverted hi rahegi
✅ Sahi! Focus se door, object paas aane pe screen image lens se door aur badi banti hai, par real aur inverted hi rehti hai.
(b) Chhoti hoti jaayegi
❌ Galat: Distance ka asar reverse kiya. Paas laane pe screen image badi hoti hai.
(c) Erect ho jaayegi
❌ Galat: Focus ke andar wali baat yahan lagayi. Focus se door, image inverted hi rehti hai.
(d) Same size rahegi
❌ Galat: Distance ka asar ignore kiya. Object paas, to image badi aur lens se door.
📚 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
Wahi magnifying glass, wahi lens… par doori badli toh tasveer badal gayi. Paas se chhote akshar bade dikhe, aur door se poori khidki deewar pe utar aayi.
Zindagi bhi aisi hi hai, beta. Kabhi koi mushkil bahut badi lagti hai kyunki hum use bahut paas se dekh rahe hote hain. Ek kadam peeche ho kar dekho, poori tasveer saaf dikhegi. Aur kabhi chhoti-chhoti baaton ko paas se, dhyaan se dekhna padta hai, tabhi woh samajh aati hain. Sahi doori dhoondhna seekh lo: padhai mein bhi, zindagi mein bhi. ✨
— CharuMam
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