GyansetuVidya ka pul
CHAPTER 09

Light — Reflection and Refraction

Overview

Roz aaina dekhte ho, paani mein straw mudhi hui dikhti hai, lens se cheezein paas-lambti dikhti hain — yeh sab roshni ke khel hain. Is chapter mein hum seekhenge ki light kaise reflect hoti hai (mirrors), kaise refract hoti hai (lenses aur glass), aur mirror/lens equations se image ki position, size aur nature kaise nikalte hain. Board exams mein yeh sabse zyada numerical-wala chapter hai — formulas yaad, signs ka dhyaan!

Key Concepts

  • 01Laws of reflection: (1) angle of incidence = angle of reflection; (2) incident ray, reflected ray aur normal ek hi plane mein hote hain.
  • 02Concave mirror light ko converge karta hai — image real & inverted ya virtual & erect ho sakti hai (object ki position par depend karta hai).
  • 03Convex mirror hamesha virtual, erect aur diminished image banata hai — isliye vehicles ke rear-view mirrors mein use hota hai.
  • 04Refraction: light jab ek medium se doosre mein jaati hai to uski speed badal jaati hai, isliye path mud jaata hai.
  • 05Snell's law: n = sin i / sin r = c/v — refractive index batata hai medium light ko kitna slow karta hai.
  • 06Convex lens converging hota hai, concave lens diverging — lens ke do focus hote hain, dono taraf.
  • 07Magnification batata hai image badi hai ya chhoti, ulti hai ya seedhi.

Important Formulas

Mirror formula: 1/v + 1/u = 1/f
Mirror magnification: m = h'/h = -v/u
Lens formula: 1/v - 1/u = 1/f
Lens magnification: m = v/u
Refractive index: n = c/v
Power of a lens: P = 1/f (f metres mein), unit dioptre (D)

Practice Questions

  1. 01Ek concave mirror ki focal length 10 cm hai. Object pole se 15 cm par hai — image ki position aur nature nikalo.
  2. 02Ek lens ka power +2.5 D hai. Focal length aur lens ka type (convex/concave) batao.
  3. 03Glass ka refractive index 1.5 hai — usme light ki speed kitni hogi? (c = 3 × 10⁸ m/s)
  4. 04Object ko convex lens ke focus par rakho — image kahan banegi? Ray diagram se samjhao.
  5. 05Glass (n = 1.5) aur water (n = 1.33) — kis medium mein light fast chalegi aur kyun?
Summary

Mirrors reflection ka rule follow karte hain (i = r), lenses refraction se image banate hain. Mirror formula 1/v + 1/u = 1/f aur lens formula 1/v - 1/u = 1/f se image position milti hai. Convex mirror ka image hamesha virtual, erect, chhota — rear-view mirror. Power P = 1/f dioptre mein — sign batata hai lens converging (positive) hai ya diverging (negative).

Handwritten notes
Handwritten — class notes
Handwritten — class notes
Diagrams + workings
Diagrams + workings

Chapter-end quiz

5 MCQs

Option tap karo — turant pata chalega sahi ya galat, explanation ke saath.

  1. Q1.Plane mirror ki focal length kitni hoti hai?

  2. Q2.Convex mirror hamesha kaisi image banata hai?

  3. Q3.Refractive index nikalne ka formula kya hai?

  4. Q4.Ek lens ki focal length −25 cm hai. Power kitni hogi?

  5. Q5.Mirror ke liye magnification ka formula kya hai?

Is chapter ka doubt?
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