2.5.13 · D5 · HinglishOptics
Question bank — Newton's rings — derivation
2.5.13 · D5· Physics › Optics › Newton's rings — derivation
True or false — justify
Har line ek claim hai. Aur reason dene ke baad hi reveal karo — decide karo true/false.
The centre of the pattern in reflected light is dark.
True — contact par hai, geometric path zero hai, lekin exactly ek reflection (air→glass) ek flip add karta hai, isliye → destructive. Dekho Stokes' relations & phase change on reflection.
Agar aap wohi setup transmitted light mein dekhein, toh centre phir bhi dark hoga.
False — transmitted light mein koi odd flip nahi hota (dono transmissions aur internal path mein koi net half-wave nahi hai), isliye centre bright hota hai aur poora pattern reflected wale ka complement hota hai.
Rings equally spaced hote hain, jaise ruler par marks.
False — , isliye radius ki tarah badhta hai; bahar ki taraf jaate hue consecutive rings aur paas aa jaate hain.
Lens ke radius of curvature ko double karne se rings bade ho jaate hain.
True — , isliye ek flatter (bada ) lens air wedge ko zyada slowly spread karta hai aur rings expand ho jaate hain.
Air gap ko wale liquid se bharne par rings expand ho jaate hain.
False — , isliye bada har ring ko se shrink kar deta hai; yahi shrinkage woh method hai jisse liquid ka index measure hota hai (dekho Refractive index measurement).
Red ki jagah blue light use karne par rings chote ho jaate hain.
True — , aur blue ka chhota hota hai, isliye saare rings centre ki taraf contract ho jaate hain.
Agar flip nahi hota, toh bright aur dark rings apni jagah badal leten.
True — flip exactly wahi cheez hai jo "usual" (normally bright) ko dark condition mein convert karta hai; ise hataao toh centre bright ho jaata hai, orders invert ho jaate hain.
Ek dark ring ka order number batata hai ki usme kitne half-wavelengths ka path difference hai.
Centre ke liye False, baaki jagah roughly true — dark rings ke liye , isliye term ke whole wavelengths count karta hai, flip pehle se condition mein fold ho chuka hai.
Agar pata ho toh aap sirf ek ring ke diameter se determine kar sakte ho.
Technically se true, lekin practically false — uncertain contact point kisi bhi single ko kharab kar deta hai; sirf diameter differences hi trustworthy results dete hain.
Spot the error
Har line mein ek reasoning hai jisme flaw hai. Reveal karne se pehle flaw identify karo.
"Path difference hai kyunki film ki ek thickness hai."
Factor 2 missing hai — ray B film se neeche aur upar jaata hai, isliye geometric path hai; 2 drop karne par jo bhi nikaaloge woh half ho jaayegi.
" par path difference zero hai, isliye waves in phase hain aur centre bright hai."
Stokes flip ko ignore kar raha hai — ek reflection add karta hai, isliye contact par total hota hai, jo dark centre deta hai.
"Rings bahar ki taraf crowd karte hain kyunki centre se door wavelength chhoti ho jaati hai."
Galat cause — source se fix hota hai; crowding geometry se aata hai, , jo thickness (aur isliye order) ko ki tarah badhata hai.
" simplify hokar ho jaata hai, ko ke comparison mein hamesha, exactly, neglect karke."
Yeh step ek approximation hai, valid sirf isliye kyunki microns aur metres hai; yeh koi identity nahi hai, aur agar gap thick ho ya chhota ho toh yeh fail kar jaata hai.
" paane ke liye ek diameter carefully measure karo aur use karo."
Absolute diameter use karta hai — fuzzy contact centre har mein ek unknown constant add karta hai; difference method chahiye taaki woh cancel ho sake.
"Dono reflected rays se flip hote hain, isliye flips cancel ho jaate hain aur matter nahi karte."
Sirf air→glass reflection (ray B, denser medium mein jaate hue) flip hota hai; glass→air reflection (ray A, rarer medium mein) nahi hota — ek flip, do nahi, isliye yeh cancel nahi hota.
"Rings fringes of equal path length hain."
Ye fringes of equal thickness hain (isliye constant ke circles); equal thickness equal path deti hai sirf isliye kyunki path mein ke zariye map hota hai — har ring par primary invariant hai.
Why questions
Sirf fact nahi, mechanism do.
Fringes circles kyun hote hain, straight lines kyun nahi?
Air gap sirf contact point se distance par depend karta hai, isliye constant ka locus ek circle hota hai — fringes of equal thickness concentric rings ban jaate hain.
Hum radii ki jagah diameters kyun measure karte hain?
Contact point deformed hota hai aur uski exact location uncertain hoti hai; diameter usi ring ke do points span karta hai isliye koi bhi centre offset cancel ho jaata hai, unlike radius jo doubtful centre se draw hoti hai.
subtract karne se ek saath do nuisances kyun remove ho jaate hain?
Ek shifted centre har mein same constant add karta hai, aur bookkeeping term bhi ek constant offset hai; do squared diameters subtract karne se dono khatam ho jaate hain, sirf clean linear bachta hai.
Do mein se sirf ek reflecting ray ko flip kyun milta hai?
Ray A glass→air reflect karta hai (dense→rare, no flip); ray B plate par air→glass reflect karta hai (rare→dense, flip) — reflection phase tabhi flip karta hai jab higher-index medium mein enter ho (Stokes' relations).
Gap ki jagah ki tarah kyun badhta hai?
Lens ek sphere hai; axis ke paas uski surface rim se sagitta ke barabar neeche jaati hai, aur circle geometry () force karta hai ki woh dip off-axis distance ke square ke saath scale kare.
Sharp rings ke liye light monochromatic (ya filtered) kyun honi chahiye?
Alag-alag wavelengths alag-alag par satisfy karte hain, isliye unke ring sets overlap karke smear ho jaate hain; ek single har ring ko crisp rakhta hai (coherence discussed in Interference — path difference & coherence).
Newton's rings air wedge ki tarah "fringes of equal thickness" phenomenon kyun hai?
Dono ek aisi film trap karte hain jiska thickness position ke saath vary karta hai, aur har fringe constant ka contour mark karta hai — ring case mein bas circular contours hote hain Wedge fringes / air wedge ke straight ones ki jagah.
Edge cases
Boundary aur degenerate scenarios — har ek ko reason karke samjho.
Agar lens aur plate ka identical refractive index ho lekin gap ek higher index wale medium se bhara ho toh pattern ka kya hoga?
Ab plate reflection dense→rare hoga (no flip) jabki lens-bottom reflection rare→dense ho jaayega (flip) — single flip abhi bhi exist karta hai lekin doosri ray par, isliye centre dark rehta hai; jo matter karta hai woh yeh hai ki exactly ek flip hota hai.
Agar gap medium ka index dono taraf ke glass se exactly equal ho (index-matched) toh kya hoga?
Kisi bhi surface par koi index step nahi hoga, essentially koi reflection nahi aur koi contrast nahi — rings gayab ho jaate hain kyunki reflect karne aur interfere karne ke liye kuch nahi hota.
Jab bahut bada ho jaaye (centre se bahut door), toh rings eventually invisible kyun ho jaate hain?
Ring spacing ki tarah shrink hoti hai isliye resolvable width se neeche crowd ho jaate hain, aur badhti thickness eventually coherence length exceed kar leti hai, contrast wash out ho jaata hai.
Exact geometric contact point par, kya thickness sach mein zero hoti hai?
Ideally haan, lekin real glass weight ke neeche deform ho jaata hai aur dust intrude karta hai, isliye "point" ek chhota flattened patch hota hai — exactly isliye absolute radii unreliable hain aur diameter differences use hote hain.
Agar bahut chhota kar diya jaaye (sharply curved lens), toh pattern kaise change hoga?
Kyunki , chhota saare rings tiny aur tightly packed bana deta hai — wedge steep ho jaata hai, thickness ke saath quickly badhti hai, isliye orders centre ke paas jaldi pile up ho jaate hain.
Jis instant aap reflected se transmitted viewing par switch karte ho, pattern kaisa dikhta hai?
Har bright dark ho jaata hai aur vice versa — transmitted pattern exact photometric complement hota hai, bright centre ke saath, kyunki reflection mein present odd flip transmission mein absent hota hai.
White light use karne par centre par aur uske thoda bahar kya dikhta hai?
Centre sabhi colours ke liye dark hota hai (path difference har ke saath scale karta hai, destructive rehta hai), lekin pehle kuch rings coloured fringes dikhate hain kyunki har colour apni bright condition slightly alag par meet karta hai.
Recall Quick self-test
Centre par ek flip ya do, aur woh kya force karta hai? ::: Exactly ek flip → net at → dark centre. Reflected vs transmitted centre? ::: Reflected dark, transmitted bright (complementary patterns). Konsa measurement quantity centre error cancel karta hai? ::: Squared diameters ke differences, .