Ray Optics and Optical Instruments Class 12 Notes | CBSE Physics Chapter 9

Ray Optics and Optical Instruments is Chapter 9 of CBSE Class 12 Physics. This chapter covers reflection and refraction at curved surfaces, total internal reflection, lenses, prisms, optical instruments like microscopes and telescopes, and the wave optics foundation concepts. It is one of the most important chapters for both boards and competitive exams.

This chapter carries 7–9 marks. Lens maker’s formula, total internal reflection, prism formula, and optical instrument numericals are the most tested.

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Key Concepts

1. Refraction at Spherical Surfaces

n₁/u + n₂/v = (n₂ − n₁)/R (single refracting surface)

2. Lens Maker’s Formula

1/f = (n − 1)[1/R₁ − 1/R₂]

Thin lens formula: 1/v − 1/u = 1/f

Magnification: m = v/u

Power: P = 1/f (in metres); unit: Dioptre (D)

3. Total Internal Reflection (TIR)

When light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle (ic), all light is reflected back — no refraction.

sin ic = n₂/n₁ (where n₁ > n₂)

Applications: Optical fibres (communication), diamond sparkle, mirages, binoculars (Porro prisms)

4. Refraction Through a Prism

n = sin[(A + δm)/2] / sin(A/2) (at minimum deviation)

where A = angle of prism, δm = minimum deviation angle

At minimum deviation: i = e and r₁ = r₂ = A/2

5. Optical Instruments

Instrument	Magnifying Power
Simple microscope	m = 1 + D/f (D = 25 cm)
Compound microscope	m = (−L/fo)(1 + D/fe), where L = tube length
Astronomical telescope (normal adjustment)	m = −fo/fe; Length = fo + fe

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Solved Examples

Example 1

Find the critical angle for glass (n = 1.5) to air.

Answer: sin ic = 1/n = 1/1.5 = 0.667; ic = sin⁻¹(0.667) = 41.8°

Example 2

An astronomical telescope has objective focal length 100 cm and eyepiece focal length 5 cm. Find magnification and tube length in normal adjustment.

Answer: m = fo/fe = 100/5 = 20. Length = fo + fe = 100 + 5 = 105 cm

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Important Questions for Board Exams

3-Mark

1. Derive the lens maker’s formula.
2. What is total internal reflection? State conditions and give two applications.
3. Derive the prism formula for minimum deviation.

5-Mark

1. Draw a ray diagram of a compound microscope and derive its magnifying power.
2. Derive the refraction formula at a single spherical surface. Use it to derive the lens maker’s formula.

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Quick Revision Points

• Single surface: n₁/u + n₂/v = (n₂ − n₁)/R
• Lens maker: 1/f = (n−1)(1/R₁ − 1/R₂); Thin lens: 1/v − 1/u = 1/f
• TIR: sin ic = n_rarer/n_denser; needs denser→rarer and i > ic
• Prism: n = sin[(A+δm)/2]/sin(A/2)
• Telescope: m = fo/fe; length = fo + fe (normal adjustment)
• Microscope: m ≈ (L/fo)(D/fe) for large magnification

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Previous: Ch 8 — EM Waves
Next: Ch 10 — Wave Optics