Class 7 Science · Chapter 11
Short answer: Class 7 Science Chapter 11 "Light: Shadows and Reflections" mein light ka straight-line travel (rectilinear propagation), shadow formation ki conditions, pinhole camera, aur plane mirror mein reflection ke do laws (∠i=∠r, aur incident-reflected ray-normal coplanar) padhte hain — saath hi lateral inversion ka concept bhi. Yeh naye NCERT "Curiosity" book (2026-27, 12 chapters) ka Chapter 11 hai.
Class 7 Science ka ye Chapter 11, "Light: Shadows and Reflections," roshni ke basic behaviour ki neev hai jispar Class 8-9-10 ki optics (refraction, lens, human eye) build hoti hai. Ye naye rationalised NCERT "Curiosity" textbook (2026-27 session, 12 chapters) ka gyarahwan chapter hai. Yahan hum seekhenge ki light kaise straight lines mein travel karti hai, shadows kaise aur kyun banti hain, pinhole camera kaise kaam karta hai, aur plane mirrors mein reflection ke laws kya hain — lateral inversion samet. Ye concepts sirf textbook tak seemit nahi — periscope, kaleidoscope, aur car ke rear-view mirrors jaisi rozmarra ki cheezon mein bhi dikhte hain.
Chapter 11 Summary — 5 Minute Revision
Class 7 Science Chapter 11 "Light: Shadows and Reflections" — jo NCERT ke naye rationalised Curiosity textbook (2026-27 session, 12-chapter structure) ka hissa hai — light ke do fundamental behaviours ko cover karta hai: shadow formation aur reflection.
Chapter ka pehla half samjhata hai ki light straight lines mein travel karti hai (rectilinear propagation), aur isi property ki wajah se opaque objects ke peeche shadows banti hain. Shadow ka size aur sharpness object-light-screen ki relative positions par depend karta hai. Isi principle ka ek practical application hai pinhole camera, jismein image ulti (inverted) banti hai.
Doosra half plane mirrors mein reflection par focus karta hai. Do laws of reflection define kiye jaate hain — angle of incidence hamesha angle of reflection ke barabar hota hai, aur incident ray, reflected ray, normal teeno ek plane mein hote hain. Iske saath lateral inversion ka concept bhi cover hota hai, jismein mirror image mein left-right reverse ho jaate hain.
Ye poora chapter Class 8 ke baad aane wale advanced optics concepts (refraction, lenses, human eye) ki foundation banata hai, aur real-life applications jaise periscope, kaleidoscope, aur ambulance ke reversed text mein directly dikhta hai.
In-Text Questions — Solutions
Chapter ke shuru mein diya gaya intext question: 'Kya andhere kamre mein bhi shadow ban sakti hai?'
Answer: Nahi, andhere kamre mein shadow nahi ban sakti. Shadow banne ke liye light source hona zaruri hai jo object ke ek taraf se roshni de aur object us light ko block kare. Agar kamre mein bilkul light hi nahi hai (complete darkness), to koi light rays travel hi nahi kar rahi, isliye block hone ka sawaal hi nahi uthta aur shadow nahi bin sakti.
Kya transparent object ki bhi shadow banti hai?
Answer: Transparent objects (jaise clean glass, saaf paani) ki shadow practically nahi banti ya bahut halki (faint) banti hai, kyunki zyadatar light unme se through ho jaati hai. Translucent objects (jaise butter paper, frosted glass) ki halki-si blurred shadow banti hai kyunki kuch light through hoti hai aur kuch scatter/block hoti hai. Sirf opaque objects (jaise wooden board, kitab) ki sharp, dark shadow banti hai kyunki woh light ko completely block karte hain.
Ek activity mein do students ne alag-alag distance se ek hi object ki shadow banayi — ek ne torch object ke bahut paas rakhi, doosre ne door se. Result kya hoga?
Answer: Jis student ne torch object ke bahut paas rakhi, uski shadow bahut badi aur thodi blurred (fuzzy edges wali) banegi, kyunki paas se light rays zyada spread hoke object ke around jaati hain. Jis student ne torch door se rakhi, uski shadow chhoti aur zyada sharp (clear edges wali) banegi, kyunki door se light rays lagbhag parallel ho jaati hain. Ye activity dikhata hai ki light source ki distance shadow ke size aur sharpness dono ko affect karti hai.
Exercise Questions — Solutions (Q1–Q6)
Q1. Boojho ne ek chart paper par ek chhoti gend rakhi aur usko table lamp se roshan kiya. Explain karo ki uski shadow kis factor par depend karegi.
Answer: Shadow ka size aur shape kai factors par depend karta hai:
- Object aur light source ke beech ki distance — object jitna light source ke paas hoga, shadow utni badi banegi.
- Object aur screen ke beech ki distance — screen paas ho to shadow chhoti aur sharp, door ho to badi aur blurred.
- Light source ka size — chhota (point) source sharp shadow deta hai, bada source fuzzy shadow deta hai (umbra-penumbra).
- Object ka orientation — light ke saamne object ka jo surface hai uske shape jaisi hi shadow banti hai, na ki poori 3D shape jaisi.
Isliye gend ki shadow hamesha round/circular hi rahegi, chahe lamp kisi bhi angle se roshan kare — kyunki gend ka silhouette har direction se circular hi dikhta hai.
Q2. Ek plane mirror ke saamne 'AMBULANCE' shabd ulta likha hota hai. Reflection ke baad woh seedha kaise dikhta hai? Diagram ki madad se samjhao.
Answer: Plane mirror lateral inversion karta hai — left aur right reverse ho jaate hain (up-down nahi).
Jab ambulance ke aage 'ECNALUBMA' (AMBULANCE ka mirror-image) likha hota hai, to jo driver apne rear-view mirror mein dekhta hai, mirror us likhawat ko dobara laterally invert kar deta hai — do inversions milkar text ko wapas seedha 'AMBULANCE' bana dete hain.
Original word → Mirror inverts → 'ECNALUBMA' painted on vehicle → Driver's mirror inverts again → 'AMBULANCE' (readable)
Isi principle ki wajah se emergency vehicles par aage ka text hamesha mirror-image mein likha jaata hai.
Q3. Paheli ne subah 8 baje apni shadow observe ki jo lambi thi, aur dopahar 1 baje shadow bahut chhoti ho gayi. Kaaran spasht karo.
Answer: Shadow ki length Sun ki position (altitude angle) par depend karti hai.
- Subah (8 AM): Sun kshitij (horizon) ke paas kam angle par hota hai, isliye light rays lambe angle se aati hain aur lambi shadow banti hai.
- Dopahar (1 PM ke aas-paas, jab Sun almost overhead hota hai): light rays lagbhag seedhi (vertical) neeche aati hain, isliye shadow chhoti ho jaati hai.
Yehi principle sundial mein use hota hai — shadow ki length aur direction se time ka andaza lagaya jaata hai.
Q4. Do plane mirrors ko ek doosre se 90° ke angle par rakha gaya hai. Ek object dono mirrors ke beech rakha jaaye to kitne images banenge? Reasoning do.
Answer: Jab do plane mirrors ek doosre se angle θ par rakhe jaate hain, to banne wale images ki number formula se milti hai:
Number of images = (360°/θ) − 1
Yahan θ = 90°, isliye:
Number of images = (360°/90°) − 1 = 4 − 1 = 3
Toh object ke total 3 images banenge — do direct reflections (ek har mirror mein) aur ek image jo dono mirrors ke combined reflection (reflection of reflection) se banti hai.
Q5. Class mein ek activity ki gayi jisme torch ki light ek gol disc par daali gayi aur screen par shadow observe ki gayi. Jab disc ko ghumaya gaya (rotate kiya) apne hi axis par, shadow ki shape kya rahegi aur kyun?
Answer: Agar disc apni face-on axis (jo screen ke parallel hai) par ghumayi jaaye, to shadow hamesha circular hi rahegi kyunki disc ka silhouette us direction se hamesha same circle jaisa dikhta hai.
Lekin agar disc ko ek vertical/horizontal diameter ke around rotate kiya jaaye (jaise page palatna), to shadow gradually circle se ellipse mein, phir ek thin line (jab disc edge-on ho jaaye) mein badal jaayegi, aur wapas circle mein aa jaayegi ek full rotation mein.
Ye dikhata hai ki shadow ki shape sirf object ki shape par nahi, balki us direction mein object ke silhouette (outline jo light ke path ko block karta hai) par depend karti hai.
Q6. Ek periscope submarine mein kaise kaam karta hai? Do plane mirrors ka use karke iska working principle diagram sahit samjhao.
Answer: Periscope ek simple optical device hai jo do plane mirrors ka use karta hai, dono 45° ke angle par parallel rakhe jaate hain, ek doosre ke aamne-saamne, ek tube ke andar.
Working:
- Upar wala mirror surface se aane wali light (jaise samundar ki sathah se upar ka drishya) ko 45° angle par reflect karke tube ke andar bhejta hai.
- Yeh reflected light neeche tak travel karti hai aur doosre mirror (jo bhi 45° par set hai) se takrati hai.
- Doosra mirror is light ko phir se 90° reflect karke observer ki aankh tak pahunchata hai (horizontal se).
Isse observer bina apni position badle, apne se upar ki cheezein — jaise submarine ke andar rehte hue samundar ki sathah — dekh sakta hai. Yeh law of reflection (angle of incidence = angle of reflection) ka direct practical application hai.
Important Equations — Ek Nazar Me
"\n| Concept | Rule / Formula | Note |
|---|---|---|
| Rectilinear propagation | Light straight lines mein travel karti hai | Shadow aur pinhole camera ka base principle |
| Law of reflection (1st) | ∠i = ∠r (Angle of incidence = Angle of reflection) | Normal se measure kiya jaata hai |
| Law of reflection (2nd) | Incident ray, reflected ray, aur normal ek hi plane mein | Coplanarity condition |
| Multiple mirror images | Number of images = (360°/θ) − 1 | θ = mirrors ke beech ka angle |
| Lateral inversion | Left ↔ Right reverse, Up-Down same | Plane mirror ki property |
| Shadow size relation | Object screen ke paas → chhoti/sharp shadow; door → badi/blurred shadow | Distance-dependent |
↔ Table ko side me swipe karein
\n"Common Mistakes — Yahan Marks Kat te Hain
- Students shadow ko object ki 'exact 3D copy' samajh lete hain, jabki shadow sirf object ke silhouette (2D outline) jaisi hoti hai — jo angle par depend karti hai.
- Lateral inversion ko 'upside down' (up-down flip) samajh lena — jabki mirror sirf left-right reverse karta hai, top-bottom nahi.
- Angle of incidence aur angle of reflection ko surface se measure karna, jabki dono hamesha 'normal' (surface ke perpendicular line) se measure kiye jaate hain.
- Translucent aur transparent objects mein confuse ho jaana — translucent halki blurred shadow deta hai, transparent lagbhag koi shadow nahi deta.
- Pinhole camera mein image invert kyun hoti hai iska reason bhool jaana — students formula yaad rakhte hain par rectilinear propagation ka logic nahi samajhte.
- Multiple mirror images ka formula (360/θ − 1) galat angle plug kar dena, ya isse general reflection questions mein bina zarurat use karna.
Board-Style Important Questions
- School-level periodic test question: 'Define rectilinear propagation of light with a labelled diagram of pinhole camera.'
- School-level assessment question: 'State and explain the two laws of reflection with a neat ray diagram.'
- Practice question (source: school sample papers): 'Why does a ray of light bend on hitting a mirror while an ambulance's front text appears reversed?'
- Practice numerical (source: school worksheets): 'Two plane mirrors are inclined at 60° to each other. Find the number of images formed.'
- Activity-based question common in school notebooks: 'Explain with an activity how the size of a shadow changes with the position of the light source.'
Aksar Poochhe Jaane Wale Sawaal
Class 7 Science Chapter 11 'Light: Shadows and Reflections' mein kya-kya topics cover hote hain?
Is chapter mein cover hota hai: light ka rectilinear (straight-line) propagation, opaque/transparent/translucent objects, shadow formation ki conditions, pinhole camera, plane mirrors mein reflection, laws of reflection, aur lateral inversion. Yeh Curiosity textbook (2026-27 session) ka Chapter 11 hai.
Shadow banne ke liye kaun-kaun si conditions zaruri hain?
Teen cheezein chahiye: (1) ek light source, (2) ek opaque ya translucent object jo light ke path mein aaye, aur (3) ek screen jahan shadow padegi. Agar object transparent ho to shadow nahi banti kyunki light usse through ho jaati hai.
Pinhole camera kaise kaam karta hai aur ismein image ulti (inverted) kyun banti hai?
Pinhole camera ek band box hota hai jismein ek chhota pinhole hota hai jisse light enter karti hai aur opposite screen par image banti hai. Kyunki light straight lines mein travel karti hai, object ke top se aane wali rays screen ke bottom par aur bottom se aane wali rays top par padti hain — isliye image inverted (ulti) banti hai. Yeh rectilinear propagation of light ka direct proof hai.
Lateral inversion kya hai aur mirror mein ye kyun hota hai?
Lateral inversion woh phenomenon hai jismein plane mirror image mein left-right reverse ho jaate hain (up-down waise ka waisa rehta hai). Jaise agar aap apna right haath uthate ho, mirror image left haath uthata hua dikhega. Ye is wajah se hota hai kyunki mirror light rays ko unke incoming direction ke exact opposite reflect karta hai, jisse image ka orientation front-to-back reverse ho jaata hai — jo effectively left-right swap jaisa dikhta hai.
Laws of reflection kya hain?
Do laws hain: (1) Angle of incidence hamesha angle of reflection ke barabar hota hai (∠i = ∠r), aur (2) incident ray, reflected ray, aur normal — teeno ek hi plane mein hote hain. Ye laws har type ke reflecting surface (plane ya curved) par apply hote hain.
Class 7 Science 2026-27 ka syllabus 12 chapters ka 'Curiosity' book hai — is chapter ka number kya hai aur pichle syllabus se kya farak hai?
Current rationalised NCERT Class 7 Science book 'Curiosity' hai jismein total 12 chapters hain (pehle 13-chapter structure se reduce hua hai). 'Light: Shadows and Reflections' is naye Curiosity book ka Chapter 11 hai. Kuch old chapters jaise Motion and Time, Electric Current and its Effects, Acids-Bases-Salts (separate chapter), Nutrition in Plants/Animals — ye ab is naye 12-chapter structure mein alag tarike se merge/rationalise ho chuke hain, isliye purani numbering se match karne ki koshish mat karo.
Class 7 Science — Saare Chapters
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