Chapter 16 – Optics (JEE Physics)
1. Introduction to Optics
Optics is the branch of physics that deals with the behavior and properties of light.
It is broadly divided into:
- Ray Optics (Geometrical Optics)
- Wave Optics
PART A – RAY OPTICS
2. Nature of Light (Ray Approximation)
When the wavelength of light is very small compared to the size of objects,
light can be treated as rays traveling in straight lines.
3. Reflection of Light
Reflection is the phenomenon in which light returns back to the same medium after striking a surface.
Laws of Reflection:
- Angle of incidence = Angle of reflection
- Incident ray, reflected ray, and normal lie in the same plane
4. Reflection by Spherical Mirrors
Types of spherical mirrors:
- Concave mirror
- Convex mirror
Mirror Formula:
$$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$
Magnification:
$$m = \frac{h_i}{h_o} = -\frac{v}{u}$$
5. Refraction of Light
Refraction is the bending of light when it passes from one medium to another due to change in speed.
Snell’s Law:
$$n_1 \sin i = n_2 \sin r$$
6. Refractive Index
Absolute Refractive Index:
$$n = \frac{c}{v}$$
Higher refractive index means lower speed of light in that medium.
7. Total Internal Reflection
Occurs when light travels from denser to rarer medium and angle of incidence exceeds critical angle.
Critical Angle:
$$\sin C = \frac{1}{n}$$
8. Refraction at Spherical Surfaces
$$\frac{n_2}{v} - \frac{n_1}{u} = \frac{n_2 - n_1}{R}$$
9. Thin Lens
Types of lenses:
- Convex lens
- Concave lens
Lens Formula:
$$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$
Magnification:
$$m = \frac{v}{u}$$
10. Power of a Lens
$$P = \frac{1}{f} \quad (\text{f in meters})$$
Unit of power is dioptre (D).
11. Combination of Lenses
$$P_{eq} = P_1 + P_2$$
PART B – WAVE OPTICS
12. Huygens’ Principle
Every point on a wavefront acts as a source of secondary wavelets.
13. Interference of Light
Interference is the redistribution of light intensity due to superposition of coherent waves.
Path Difference:
$$\Delta = d \sin \theta$$
Condition for Maxima:
$$\Delta = n\lambda$$
Condition for Minima:
$$\Delta = (2n+1)\frac{\lambda}{2}$$
14. Young’s Double Slit Experiment
Fringe Width:
$$\beta = \frac{\lambda D}{d}$$
15. Diffraction
Diffraction is the bending of light around obstacles whose size is comparable to wavelength.
Angular Width of Central Maxima:
$$\theta = \frac{\lambda}{a}$$
16. Polarization
Polarization is the phenomenon of restricting vibrations of light to a single plane.
Only transverse waves can be polarized.
17. Important JEE Exam Tips
- Always use sign convention correctly
- Draw ray diagrams wherever possible
- Wave optics formulas are frequently tested
- Dimensional analysis helps verify answers
18. Final Revision Checklist
You have mastered Optics if you can:
- Solve mirror and lens problems confidently
- Apply Snell’s law and TIR conditions
- Handle interference and diffraction formulas
- Differentiate between ray and wave optics