Chapter 9: Ray Optics and Optical Instruments.

the genius April 08, 2026

 

Ray Optics and Optical Instruments: Class 12 Physics Chapter 9 Notes in Hindi

Ray Optics mein hum maan kar chalte hain ki light ek sidhi rekha (Straight line) mein chalti hai jise hum "Ray" kehte hain. Is chapter mein hum reflection, refraction aur bade-bade instruments jaise Telescope aur Microscope ke baare mein padhenge.

1. Reflection of Light (Pravartan)

Jab light kisi shiny surface (mirror) se takrakar wapas usi medium mein laut aati hai, toh use Reflection kehte hain.

  • Mirror Formula:

    $$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$

  • Linear Magnification ($m$):

    $$m = \frac{h'}{h} = -\frac{v}{u}$$

  • Important: Concave mirror ke liye $f$ hamesha negative $(-)$ hota hai.

2. Refraction of Light (Apvartan)

Jab light ek medium se dusre medium mein jati hai, toh wo apne raste se bhatak jati hai. Isse Refraction kehte hain.

  • Snell’s Law:

    $$\frac{\sin i}{\sin r} = \frac{n_2}{n_1}$$

  • Refractive Index ($n$):

    $$n = \frac{c}{v}$$

    (Light ki speed vacuum mein / medium mein).

    • Shutterstock

3. Total Internal Reflection (TIR)

Jab light denser medium (pani) se rarer medium (hawa) mein jati hai aur angle of incidence Critical Angle ($i_c$) se bada ho jata hai, toh light wapas usi medium mein laut aati hai.

  • Condition: Light denser se rarer mein jani chahiye.

  • Applications: Optical Fibres, Mirage (marichika), aur Heere (Diamond) ki chamak.

4. Lens Formula aur Power

Lenses do tarah ke hote hain: Convex (Converging) aur Concave (Diverging).

  • Lens Maker’s Formula:

    $$\frac{1}{f} = (n - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)$$

  • Lens Formula:

    $$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$$

  • Power of Lens ($P$):

    $$P = \frac{1}{f (\text{in meters})}$$

    (Unit: Dioptre (D))

5. Refraction through a Prism

Prism se guzarne par light 7 rangon mein toot jati hai (Dispersion).

  • Prism Formula:

    $$n = \frac{\sin \left( \frac{A + \delta_m}{2} \right)}{\sin \left( \frac{A}{2} \right)}$$

    (Yahan $A$ prism angle hai aur $\delta_m$ minimum deviation).

6. Optical Instruments (Sabse Important Section)

Boards mein inme se ek derivation pakka aata hai:

  1. Simple Microscope: Isme ek convex lens hota hai. Iska magnification $M = 1 + \frac{D}{f}$ hota hai.

  2. Compound Microscope: Isme do lenses hote hain (Objective aur Eyepiece). Ye choti cheezon ko bahut bada karke dikhata hai.

  3. Astronomical Telescope: Ye door ki cheezon (sitare, grah) ko dekhne ke kaam aata hai.

7. Top Important Questions (Chapter 9)

Aapke blog ke liye "Most Expected" questions:

Short Questions:

  1. Critical Angle kya hai? Wo angle jispar angle of refraction $90^\circ$ ho jaye.

  2. Kiski power positive hoti hai? Convex lens ki power (+) aur Concave ki (-) hoti hai.

  3. Optical Fibre kis principle par kaam karta hai? Total Internal Reflection (TIR).

Long Derivations (5 Marks):

  1. Derive Lens Maker’s Formula. (Sabse zyada pucha gaya sawal).

  2. Derive the Mirror Formula for a concave mirror.

  3. Compound Microscope ka Ray Diagram banaiye aur magnification derive kijiye.

  4. Refraction at a spherical surface ka formula ($n_2/v - n_1/u = (n_2-n_1)/R$) prove kijiye.

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