Ray Optics and Optical Instruments

part 1

Reflection and Refraction

• Reflection of light — spherical mirrors
• Mirror formula and magnification
• Refraction of light — Snell's law
• Total internal reflection and applications
• Refraction at spherical surfaces

part 2

Lenses and Prisms

• Thin lens formula
• Lens maker's equation
• Power of a lens
• Combination of thin lenses
• Refraction through a prism
• Dispersion and scattering of light

part 3

Optical Instruments

• Human eye and its defects
• Simple microscope
• Compound microscope
• Telescope (refracting and reflecting)
• Resolving power of microscope and telescope

Mirror formula

$\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$; $f = \frac{R}{2}$

★ board favourite

Magnification (mirror)

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

★ board favourite

Snell's law of refraction

$n_1\sin\theta_1 = n_2\sin\theta_2$

★ board favourite

Critical angle for TIR

$\sin C = \frac{n_2}{n_1}$ (n₁ > n₂)

★ board favourite

Lens formula

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

★ board favourite

Lens maker's formula

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

★ board favourite

Prism: refractive index (min deviation)

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

★ board favourite

Magnification (lens)

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

★ frequently asked

Power of a lens

$P = \frac{1}{f(m)}$ [unit: dioptre, D]

★ frequently asked

Lenses in contact (equivalent)

$\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}$; $P = P_1 + P_2$

★ frequently asked

Draw a labelled ray diagram of a compound microscope forming the final image at the least distance of distinct vision. Derive an expression for its total magnifying power. How does it differ from an astronomical telescope in construction and use?

Derive the relation n₂/v − n₁/u = (n₂ − n₁)/R for refraction at a single convex spherical surface, stating the assumptions made.

Derive the lens maker's equation. A convex lens of refractive index 1.5 has radii of curvature 20cm and 30cm. Find the focal length of the lens in air and when immersed in water (μ_water = 4/3).

Derive the relation between the refractive index of a prism, its angle A and the angle of minimum deviation δ_m. Trace the path of a ray through a prism at minimum deviation.

Define critical angle. Derive the relation between critical angle and refractive index. Give two applications of total internal reflection.

Draw a labelled ray diagram of a refracting astronomical telescope in normal adjustment. Define its magnifying power and derive its expression. Write two limitations of a refracting telescope over a reflecting telescope.

A concave mirror produces a virtual image of the same size as the object. Where must the object be placed?

Two thin convex lenses of focal lengths f₁ and f₂ are placed in contact. Derive an expression for the focal length of the combination. Hence define the power of a lens and its SI unit.

Using the mirror formula, derive the relation for the magnification produced by a concave mirror. An object 4 cm high is placed 30 cm in front of a concave mirror of focal length 20 cm. Find the position, nature and size of the image.

The refractive index of the material of an optical fibre is 1.5. Explain, with a diagram, how light travels through the fibre by total internal reflection even when the fibre is bent.

A ray of light passes from a denser medium to a rarer medium. Define critical angle for this pair of media and derive its relation with refractive index. Draw a diagram to show refraction when the angle of incidence exceeds the critical angle.

A convex lens is combined with a concave lens of equal focal length. What is the power of the combination?

How many marks is Ray Optics and Optical Instruments worth in the Class 12 board exam?

Around 10 marks of the 70-mark CBSE Physics theory paper, based on the official unit-wise weightage. Highest-marks chapter! Lens maker's equation + numerical practice daily.

Is Ray Optics and Optical Instruments easy or hard?

It's rated hard — one of the tougher chapters in Class 12 Physics. Most students need about 7 hours to cover it well.

What are the important topics in Ray Optics and Optical Instruments?

The chapter covers 16 NCERT topics in 3 areas: Reflection and Refraction; Lenses and Prisms; Optical Instruments.

What questions come from Ray Optics and Optical Instruments in board exams?

Between 2018–2024, CBSE board papers asked questions from this chapter on Compound Microscope, Refraction at Spherical Surface, Lenses, Prism — 1- to 5-mark questions.