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Build A DIY Radio Telescope From Scratch To Detect Hydrogen Line Emissions

Build A DIY Radio Telescope From Scratch To Detect Hydrogen Line Emissions — a free intermediate-level guide covering build a diy radio telescope from...

89 min read8 chaptersintermediate

What you will learn

  1. Basic Physics Ka Funda, Varna Sab Zero
  2. Dish Market Se Lau, Bheek Mat Maango
  3. Signal Pakdo Bhai, LNA Laga Ke Scene On Karo
  4. SDR Dongle Laga Ke System Cool Karo
  5. Code Chalao, Software Se Noise Kaato
  6. Asmaan Ki Taraf Point Karo, Andha Dhun Nahi
  7. Pehla Scan Lo, Darne Ka Nahi
  8. Data Pe Chhaapo Laga Ke Milky Way Map Karo

1. Basic Physics Ka Funda, Varna Sab Zero

Abe sun, agar tu soch raha hai ki telescope matlab wo chashma jismein se chand-tare dekhte hain, toh tu galat jagah aa gaya hai. Utha apna boot aur nikal yahan se. Dekh bhai, real story yeh hai: raat ke 3 baje jab tu baniyan pehen ke code kar raha hota hai aur tabhi Sharma ji ka beta IIT crack karke neighbourhood mein apna boombox chala raha hota hai, tab tera reaction kya hota hai? "BC kya shor kar raha hai saala!" Lekin agar tu soche, toh us Sharma ji ke bete ka shor aur universe ka sabse bada shor ek hi cheez hai — Energy. Aaj hum nahi ban rahe wo chashme wale telescope. Hum ban rahe hain Radio Telescope. Ye aankh se nahi dekhta, ye sunta hai. Aur kya sunta hai? Universe ki sabse basic, sabse zyada milne wali aawaz — Hydrogen ki aawaz. Agar is chapter ka basic physics skip kiya, toh baaki ke 7 chapters mein tu wahi karega jo local train mein bina ticket chadne wala karta hai — andha dhun andar ghusna aur phir TT ke haath pakad ke bahar aana. Samjha? Chal, collar theek kar, chai pe le, aur dhyan se sun. Asli Funda (Core Concepts) Radio Frequency aur Wavelength: Autorickshaw ka Principle Saale, physics ki kitab mein formula dekh ke tera dimag ghoom jata hai, par funda ekdum simple hai. Tu ek autorickshaw mein baith. Agar rickshaw wala meter dabata hai, toh meter ki sui (needle) kaise move karti hai? Hava mein aawaz (wave) aati hai, aur meter usko detect karta hai. Radio frequency (RF) bhi bas hawa mein travel karti hai, par aawaz se bahut zyada fast aur door tak jaati hai. Ab wavelength kya hai? Bhai ye aise samajh, jaise Mumbai local train ke dabbe ka length. - Agar dabba chhota hai (short wavelength), toh wo bahut tez ghoomta hai, energy zyada, par raste mein kisi bhi chhote haadse (obstacle) pe takrayega aur ruk jayega. Ye hai High Frequency (jaise visible light ya X-ray). - Agar dabba bahut bada hai (long wavelength), toh wo dheere chalta hai, energy kam hai, par raste ki building ko tod ke, baarish mein bina ruke seedha destination pohonch jata hai. Ye hai Low Frequency (Radio waves). 💡 Pro Tip: Yaad rakh, frequency aur wavelength ka ulta proportion hai. Frequency badhi, toh wavelength kam hui. Simple math: c = f × λ (Speed of light = frequency × wavelength). Light ki speed constant hai, toh jo tera frequency badhega, teri wavelength ghategi. Period. Hum jo signal catch karne wale hain, wo 1420.405 MHz pe aata hai. Ye frequency aisi hai ki iski wavelength lagbhag 21 centimeters hoti hai. Isliye isko 21cm Hydrogen Line bolte hain. 21cm …

2. Dish Market Se Lau, Bheek Mat Maango

Abe sun, agar tu raste pe chal raha hai aur tujhe lag raha hai ki tera startup flop ho gaya toh koi baat nahi, lekin agar tune Chapter 1 skip kiya hai na, toh tera dimaag ka RAM crash ho jaayega. Pichle chapter mein humne 21cm Hydrogen Line ka sach samjha, system ke total damage ka hisaab kitaab (Total Damage:) rakha, aur ye bhi dekha ki Beamwidth aur Sensitivity kya hoti hai. Ab theory band, asli kaam shuru. Aaj hum woh cheez uthayenge jo is poore setup ki jaan hai—Dish Antenna. Dekh bhai, radio telescope banana ekdum arranged marriage jaisa hai. Ek taraf tu hai (ter laptop aur SDR), doosri taraf woh hai (Milky Way galaxy ka hydrogen). Beech mein jo rishta jodta hai na? Woh hai tera DTH dish. Agar ye dish khoti nikli, toh tera poora setup ekdum Sharma ji ke bete ki maths marks jaisa—bekaar aur sharmnak. Saale, kisi bade scientist ne bola tha, "Give me a big enough dish and I will move the galaxy's signals into my laptop." (Thoda modify kar liya, chill). Chalo, market uthte hain. Asli Funda (Core Concepts) 1. DTH Dish: Bheek Mat Maango, Jugaad Karo Tu soch raha hoga ki "Bhai, main toh ISRO banunga, ekdum saaf-suthra naya 5-foot ka dish launga." BC, aukaat mein reh. Naya commercial dish khareedega toh tera bank account ro lega. Asli gully ka don kabhi naya khareedta nahi, woh jugaad karta hai. Humein 1420.405 MHz (21cm line) pakadni hai. Is frequency par signal itna weak hota hai ki agar tera dish chhota hoga, toh space ka noise bhi nahi pakdega. Minimum 60 cm to 90 cm ka dish chahiye. Par sabse best hai 1.2 meter (4 feet) se 1.5 meter (5 feet) wala. 💡 Pro Tip: Tata Sky, Airtel, ya Videocon ka purana DTH dish perfect rehta hai. Inka diameter 60cm se 90cm hota hai. Agar tujhe bada chahiye, toh Doordarshan (DD Free Dish) wale solid 1.2 meter ke dish utha le. Woh saste aur tuff hote hain. Market kahan se lau? Scrap wale dekh le. Mumbai hai toh Bhendi Bazaar, Delhi hai toh Sadar Bazar, kisi bhi local TV repair shop pe ja. Wahan dher saare chhat girne wale ya upgrade karne wale logon ke purane dish pade hote hain. Bhai, 200-300 rupaye de ke 90cm ka solid dish utha le. Kisi ki nahi rahi, teri ho jayegi. Bas ek dhyan rakhna—dish par gaajar-matar (rust) na ho. Thoda sa rust theek hai, par agar panna (mesh) hi toot gaya hai, toh usko mat lena. 2. Parabolic Dish Ka Maths: Focal Length Nikalo Ab tu dish le aaya. Dekha, ek gol sa bartaan hai. Par isko kaise use karein? Yahan se asli physics aati hai, …

3. Signal Pakdo Bhai, LNA Laga Ke Scene On Karo

Abe sun, tera bhai local train mein 6 baje ki Borivali local mein travel kar raha tha. Ek bhaiya aage baith ke phone pe loudspeaker pe podcast sun rahe the. Volume full. Saara coach sun raha tha ki "startup ecosystem kaise fail hota hai." Ek aadmi ne bola, "Bhaiya, earphone lagao na." Bole, "Mera phone ka speaker weak hai, earphone nahi hai." Toh ek tapori ne peeche se chillaya, "Abe speaker weak hai toh volume badhaata hai saala, amplifier laga na chahiye tha!" Sach bol raha tha wo tapori. Agar source weak hai, toh tujhe usko pakadne ke liye, sunne ke liye, ekdamn apne system mein "amplification" chahiye. Saale, last chapter tak tu ek DTH dish utha ke laaya. Sharma ji ka beta bol raha tha "ye toh ghar mein sab lagate hain." Par BC, Sharma ji ka beta Netflix dekhta hai dish se, tu Milky Way ki aawaz sunta hai. Tu jo signal dhoond raha hai na—wo 1420.405 MHz wala 21cm Hydrogen Line—wo itna weak hai ki agar wo sound hota, toh tu Mumbai ki local train ke shor mein ek machhar ke rone ki aawaz sunne ki koshish kar raha hota. Agar tune dish laga li, SDR jod liya, par beech mein signal ko boost karne wala 'amplifier' nahi lagaya... toh tera pura setup bekar hai. Tera laptop screen pe bas garbage noise dikhega aur tu sochega "mera telescope kharab hai." Nahi bhai, tera dimaag kharab hai. Aaj hum signal ko pakdnge, usko bina noise badhaye amplify karenge, aur dish ke saath ekdum jugaad se connect karenge. Chapter 3: "Signal Pakdo Bhai, LNA Laga Ke Scene On Karo." Collar pakad, chal. --- Asli Funda (Core Concepts) Dekh bhai, asli scene samajh. Dish tere ghar aa gayi. Par dish sirf metal ka dabba hai. Wo akela kuch nahi karega. Dish ka kaam hai aasmaan se aate hue weak radio waves ko collect karke ek point pe focus karna. Lekin wo focused signal abhi hawa mein hai. Usko pakadne ke liye tujhe ek "net" chahiye. Us net ko bolte hain Feedhorn. Aur us net ke foran baad, us signal ko itna zor dena padega ki wo tere SDR dongle tak pahunch sake. Is zor ke liye chahiye LNA (Low Noise Amplifier). Breakdown karte hain, ekdum bhand pe. 1. Feedhorn: Signal Ka Catcher Bhai ye aise samajh, jaise tu biryani order karta hai aur delivery boy tujhe bahar aake bulata hai. Dish tera ghar hai, feedhorn tera darwaza hai. Aasmaan se signal aayega, dish pe thokega, aur feedhorn ke andar ghuske coaxial cable mein utar jaayega. Feedhorn ka kaam simple hai: dish se reflect hoke aate hue waves ko pakadna aur unhe ek narrow tube mein bhejna jahan humara receiver (LNA) …

4. SDR Dongle Laga Ke System Cool Karo

Abe sun, kal raat 3 baje ek bande ne message kiya — "Bhai, LNA lag gaya, dish set ho gayi, par screen pe kuch aa hi nahi raha. Kya scam hai?" Maine pucha, "Dongle aur software setup kiya?" Wo bola, "Arey wo toh kal karunga." BC! Engine lagadi hai, tyre lagaye, body paint kar di, par steering wheel baitha nahi. Gaadi chalayega kaise, dimaag se? Saale, antenna banake LNA lagane se radio telescope nahi ban jaata. Wo analog signal abhi bhi hawa mein hai. Tu usko pakad toh raha hai, par samajh nahi raha. Isliye Chapter 4 aaya hai — SDR Dongle Laga Ke System Cool Karo. Ye wo chapter hai jahan analog ki duniya digital ki duniya se aati hai milti. Agar yaha galti ki, toh pehle 3 chapter ka mehnat pe moot dega tu. Dimaag lagaa, aur padh. Asli Funda (Core Concepts) Analog ki Aawaz, Digital ki Bhasha Dekh bhai, scene clear kar. Tere paas LNA hai. Dish se signal aaya, feedhorn ne pakda, LNA ne 1.42 GHz ki weak signal ko thoda boost kar diya. Ab ye signal tere paas aa raha hai coaxial cable ke through. Lekin ye signal abhi bhi analog hai. Continuous waveform hai. Tu usko laptop mein daalke dekhega? Laptop bhi gaali dega. Laptop sirf 0 aur 1 samajhta hai. Usko bits chahiye. Ye conversion kaam karta hai jaise tu biryani order karta hai. Hotel wala biryani bana raha hai (analog signal), par Zomato wala usko tere ghar tak deliver karta hai. SDR dongle wo Zomato delivery boy hai jo analog biryani ko digital packets mein convert karke tere laptop ke table pe rakh dega. 💡 Pro Tip: SDR ka matlab hi hai Software Defined Radio. Pehle ke zamaane mein radio banane ke liye 50 alag-alag ke circuits aur capacitors jodne padte the. Ab ek chota sa USB dongle lagao, baaki sab software karega. Jugaad level: God. RTL-SDR: Sasta, Sundar, Tikau Saale, market mein hazaro ke dongle milenge. HackRF, Airspy, SDRplay... BC itne paise de dunga tu ghar nahi aayega. Tujhe shuruat karni hai. Tere liye RTL-SDR hai. Pichhle chapter mein humne iska naam liya tha, ab isko kholte hain. RTL-SDR originally ek DVB-T TV tuner dongle tha. Yaad hai tune 2010 mein TV pe match dekhne ke liye wo chota sa USB antenna wala dabba liya tha? Wahi. Kuch hackers aur radio geeks ne socha, "BC isme toh RTL2832U naam ka chip hai, isko thoda hack kar ke hum wideband radio receiver bana sakte hain!" Aur ban gaya. 20-30 dollars mein tu 25 MHz se 1700 MHz tak kuch bhi sun sakta hai. Tera 1420.405 MHz toh iske aadhe pahaad mein bhi aata hai. Lekin ek problem hai. RTL-SDR ka ADC (Analog …

5. Code Chalao, Software Se Noise Kaato

Abe sun, raat ke 3 baje jab tu code debug kar raha tha aur 42 errors aaye the, tab tune kya kiya tha? Rona aaya tha na? "Bhaiya ye kyun nahi chal raha, BC main toh line by line copy kiya hai!" Saale, sharma ji ka beta jab top karta hai toh usko lagta hai wo Einstein hai, par jab tu apna SDR dongle USB mein daalta hai aur screen pe sirf snowstorm (barish) jaisa garbage dekhta hai, tab tera dimaag kharab ho jaata hai. Pichle char chapter mein tune hardware joda, dish uthai, LNA lagaya, SDR dongle ko system se connect kiya. Ab hardware tera ready hai, biceps ban gaye hain. Par beta, bina dimaag ke biceps ka koi kaam nahi. Abhi tera system andha hai. Usko aankh deni padegi, aur us aankh ka naam hai Software. Agar is chapter tune dhyan se nahi padha, toh tera lakh ka hardware ekdum bekar jaayega. Sach mein, kya maa chd raha hai sacchi mein? Focus kar! Asli Funda (Core Concepts) Dekh bhai, ye aise samajh. Tu local train mein Virar se Churchgate ja raha hai. Train mein bheed hai, koi bol raha hai, koi chaye peela raha hai, koi phone pe jhagda kar raha hai. Ye sab tere kaan mein aa raha hai. Par tere ko sirf "Pani Puri wala bhaiya" ki aawaz sunni hai jo platform pe chillaa raha hai. Tere ko baaki saari aawaz ko (noise) ignore karna padega, aur sirf uss ek aawaz (1420.405 MHz signal) ko pakadna padega. SDR (Software Defined Radio) ka funda exactly yahi hai. Tera LNA aur Dish space se signal laa rahe hain, par saath mein wo local FM stations, mobile towers, WiFi routers, aur tera padosi ka microwave oven ka radiation bhi utha rahe hain. Is garbage ko kaatne ke liye tere paas do asli hathiyar hain: SDR (SDR Sharp) aur GNU Radio. 1. SDR (SDR Sharp) - The "Jugaad" Setup Ye Windows pe chalne wala sabse easy software hai. Bina muchh ke seedha kaam pe lagta hai. Jaise tu ghar pe Maggi banata hai 2 minute mein—fast, easy, par thoda basic. Setup kaise kare? 1. RTL-SDR dongle USB mein daal. 2. Zadig driver install kar (Windows isko nahi pehchanta, Zadig usko bataata hai ki "abe ye dost hai, ghusne de"). 3. SDR download kar aur rtl-sdr.dll file uske folder mein daal. 4. Run kar. Jab SDR khulega, toh tu ek waterfall graph dekhega. Ye graph basically time aur frequency ka plot hai. Tera kaam hai frequency ko exactly 1420.405 MHz pe set karna. 2. GNU Radio - The "Biryani" Setup Agar SDR Maggi hai, toh GNU Radio proper Lucknowi Biryani hai. Thoda time lagta hai banane mein, par ek baar ban …

6. Asmaan Ki Taraf Point Karo, Andha Dhun Nahi

Abe sun, kal raat 3 baje ek bhai code debug kar raha tha. Error "Object reference not set" aa raha tha. Do ghante baitha, ro raha tha, dimaag kha gaya. Phir ekdum se usne screen ki taraf dekha — monitor ka power cable hi nahi laga tha. BC, monitor hi off tha, toh error kahan se dikhe? Yeh exactly tera scene hai bhai. Pichle 5 chapter mein tune itna kuch kiya — Dish uthayi, LNA lagaya, SDR dongle ka setting kiya, code se noise kaata. System ekdum redy hai. Lekin agar tune dish ko aise hi andha-dhun andheri taraf point kar diya, toh tera pura setup uss monitor jaisa hai jiska power cable hi nahi laga. Galaxy se signal aa raha hai, tera LNA macha raha hai, par teri dish kahin aur dekh rahi hai. Kya maa chd raha hai sacchi mein? Saale, radio telescope ka matlab ye nahi ki dish utha ke asmaan ki taraf kar dega toh aliens ka phone aa jaayega. Asmaan itna bada hai, aur teri dish ka Beamwidth itna chhota hai. Tune pichle chapter mein beamwidth ka funda toh padha hoga? Agar nahi padha, toh wapas jaake padh, abhi mat aana yahan. Agar tune direction nahi pakdi, toh 1420.405 MHz ka signal tujhe milne se pehle tera patience mar jaayega. Tu radio telescope ka data lega ya noise ka salad? Aaj hum iska system set karenge. Dish ko galaxy ke hisaab se track karna seekhenge. Manual ya motorized Alt-azimuth mount banaunga, Stellarium se target coordinates nikalna sikhayunga, aur Milky Way ke galactic plane ko point karna sikhayunga. Ready? Nahi? Padh dubara BC. Asli Funda (Core Concepts) Dekh bhai, Asmaan ko dekhne ke do tareeke hote hain. Ek wo hai jo teri mummy karti hai — "Beta upar dekh, koi toh star dikhta hoga?" — aur ek wo hai jo ISRO wale karte hain (precise coordinates ke sath). Humko ISRO wali precision chahiye, kyunki 21cm hydrogen line ke signals itne weak hote hain ki thodi si bhi direction galti ki, toh signal gaya tel lene. 1. Galactic Plane Aur Teri Dish Ka Rishta Milky Way galaxy — jisme hum sab rehte hain — ek flat disc jaisa hai. Jaise teri mummy ki roti belne ka belan, bas waise flat. Aur is disc ke andar, idhar-udhar bharta hua hydrogen gas. Ab 21cm Hydrogen Line ka signal sabse zyada kahan se aayega? Jahan gas sabse zyada hogi. Aur gas sabse zyada kahan hai? Galaxy ke plane mein, yaani us disc ke beech mein. 🎯 Key Insight: Galactic plane milky way ka wo hissa hai jahan se hydrogen signals sabse strong aate hain. Agar dish ko plane ke along scan karega, toh jackpot. Bahar point kiya, toh zero. Isliye, …

7. Pehla Scan Lo, Darne Ka Nahi

Abe sun, tera startup ka sapna kaisa chal raha hai? Shankar tapri wale ne kal bola tha kuch logon ne chai pe 5000 rupaye udhaar leye the aur bhaag gaye. Tu toh bhaag nahi raha na? Chapter 6 mein tune SDR software aur noise filtering ka pura kachcha saaf kar diya. Code likha, garbage signal kaato, system ready hai. Ab tera laptop aur SDR dongle ekdum ready hai, dish pe feedhorn lag gaya, LNA ne apna kaam shuru kar diya. Lekin saale, sirf system banane se kuch nahi hota. Jaise tere Papa ne scooter kharidi thi aur pehle din hi ghar ke compound ki deewar tod di thi — kyunki unhone practice nahi ki thi. Pehla ride hamesha risky hota hai. Aaj humara pehla scan hai. Aaj humara pehla dummy observation run hai. Darne ka nahi, par dimag lagane ka. Aaj hum real-time data collect karenge, system ko noise diode se calibrate karenge, aur data ko CSV ya FITS format mein save karenge. Agar is step mein tu ghoosa, toh pura Module 1 se 6 ka paani. Chal, collar tight kar, chai ka last ghoot lu, aur shuru karte hain. Asli Funda (Core Concepts) Pehla Scan: Darna Kyon? Bhai, dekh. Tu jab pehli baar local train ka gate pakad ke andar ghusta hai, toh lagta hai ki koi muh de ke nikal dega. Par ek baar andar ghuse, toh pata chalta hai ki bas khade raho, koi kuch nahi karega. Radio telescope ka pehla scan bhi aise hi hai. Tune dish ko assemble kiya, LNA lagaya, SDR connect kiya, code likha. Ab screen pe dekh — kya aa raha hai? Shuru mein kuch dikhega nahi. Bas ekdum flat line, ya phir hawa ka noise. Saale, dar mat. Asmaan se 21cm hydrogen line ka signal itna weak hota hai ki pehli baar mein wo tujhe chikke pe chhapke nahi maarega. Pehla scan sirf yeh check karne ke liye hota hai ki tera system zinda hai ya mar gaya. Hum dummy observation run karenge. Matlab, kisi aise hisse pe dish point karenge jahan humein pata hai ki signal hai ya nahi, aur dekhenge ki kya system usko record kar paa raha hai. ⚠️ Common Mistake: Pehla scan mein hi Milky Way ke center pe dish point karke "Bhai kahan hai signal?" chillaana. Saale, center pe itna zyada signal aur confusion hota hai ki tera naya system wahi confuse ho jayega. Shuruat side se kar, jahan background thoda calm ho. Calibration: Noise Diode Ka Khel Ab aata hai sabse important hissa — Calibration. Bina iske tera pura data ekdum bekaar hai. Jaise tere exam mein 40 marks aaye, par check karne wala master ji 10 marks apne chashme ke hisaab se kaat le. …

8. Data Pe Chhaapo Laga Ke Milky Way Map Karo

Abe sun, Sharma ji ka beta IIT crack karke US chala gaya, tera startup funding nahi mila, aur tera crush ne dusre ladke ke saath reel banayi. Lagta hai sab khatam? BC, asli khatma toh tab hota jab tu surrender kar de! Tu 7 chapters survive kar gaya — dish utha, LNA lagaya, SDR config kiya, dish ko Milky Way ki taraf point kiya, aur scan bhi liya. Tu abhi tak zinda hai, matlab tu sale mein genuinely kamaal ka banda hai. Aaj tera final exam hai. Aaj hum us garbage data mein se asli Milky Way ka map nikalenge. Aaj tu scientist banega, warna raste ka nautanki banke reh jaayega. Bhai, jab tu Pehla Scan Lo, Darne Ka Nahi wala step complete kiya tha, tune ek bloody messy spectrum dekha tha. Woh file tere laptop mein aise padi hai, jaise local train ke dabbe mein 6 baje ki bhheed mein ek bindass aadmi so raha ho — poori tarah dhakela hua, chaotic, aur samajh nahi aa raha kahan se pakad ke bahar nikaale. Aaj usko nanga karke saamne rakhenge. Dekh, asli kaam shuru hota hai ab. Data collect karna aasan tha, usme se galaxy ka raaz nikalna asli gully ka business hai. Agar ye step fail gaya, toh pehle 7 chapters ka effort sab ekdum zero. Isliye dimaag lagaa, chai pi le, aur code pe chhaapo laga. Asli Funda (Core Concepts) 1. Data Cleaning: Kachra Saaf Karo Bhai Tere paas jo .dat ya .csv file SDR software (jaise SDR ya GQRX) se nikali hai, woh ekdum bekaar hai. Usme noise hai, interference hai, aur tere padosi ke microwave oven ki radiation bhi shamil hai. Bhai ye aise samajh, jaise tu biryani order karta hai aur usme kisi ne elachi daal di. Elachi nikal ke biryani khani padti hai na? Wahi yahan hai. Humein apne 21cm Hydrogen Line (1420.405 MHz) ke aas-paas ka data lena hai, aur baaki saara kachra cut karna hai. ⚠️ Common Mistake: Pura frequency range plot karne ki koshish mat karna. Tera graph ekdum flat aur boring dikhega. Humein sirf 1420 MHz se 1421 MHz ke beech ka hissa chahiye, kyunki wahi hydrogen ki aawaz aati hai. Python Code Kaise Lagaana Hai: Tu Python use karega. numpy aur matplotlib tere hathiyar hain. Agar Python nahi aata, toh YouTube dekh, but yahan code de, copy-paste kar, aur chala. Chala? Ekdum mast! Tune ab ek graph dekha hoga jisme ek bump (danda) dikhega. Woh danda hai teri Milky Way ka hydrogen. Agar bump nahi dikha, toh tera dish theek nahi tha ya scan kharab tha. Wapas ja, Module 7 dekh. 2. Doppler Shift: Galaxy Ka Speed Meter Ab aata hai asli khel. Doppler Effect ka funda yaad hai? …

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