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Cheat sheet · Class 12 · CBSE · Science Physics

Ray Optics and Optical Instruments cheat sheet

The whole chapter on one page: key formulas, facts and the points to remember, for quick revision before a test.

CBSE Class 12 · Physics

Cheat sheet

Chapter 9Ray Optics and Optical Instruments

Mirror: 1/v + 1/u = 1/f. Lens: 1/v − 1/u = 1/f.

1. Mirrors & Lenses

1v+1u=1f\dfrac1v + \dfrac1u = \dfrac1f (mirror)f=R2f = \dfrac R2
1v−1u=1f\dfrac1v - \dfrac1u = \dfrac1f (lens)P=1fP = \dfrac1f (dioptre)

Lens maker's: 1f=(μ−1)(1R1−1R2)\dfrac1f = (\mu - 1)\left(\dfrac1{R_1} - \dfrac1{R_2}\right).

2. Refraction

μ=sin⁡isin⁡r\mu = \dfrac{\sin i}{\sin r}sin⁡C=1μ\sin C = \dfrac1\mu
  • Total internal reflection: optical fibres, diamonds, mirages.
  • Prism: μ=sin⁡A+Dm2sin⁡A2\mu = \dfrac{\sin\frac{A + D_m}{2}}{\sin\frac A2}.

3. Optical Instruments

InstrumentMagnification
Simple microscopem=1+Dfm = 1 + \dfrac Df
Compound microscopem=Lfo⋅Dfem = \dfrac{L}{f_o}\cdot\dfrac{D}{f_e}
Telescope (normal adjustment)m=fofem = \dfrac{f_o}{f_e}, length fo+fef_o + f_e

4. Key Points

  • Combined lenses: P = P₁ + P₂.
  • Real image: negative v for mirrors (sign convention).
  • Blue sky: scattering.
  • Reflecting telescopes avoid chromatic aberration.

Remember: Mirror & lens formulas | TIR | microscope, telescope

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