NEB Class 12 Physics Nature and Propagation of Light Notes in PDF Complete Handwritten. Physics Notes 2081: All Chapters | New Curriculum | Class 12 Physics Notes download.
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NEB Class 12 Physics Nature and Propagation of Light Note Handwritten in PDF
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Wave Nature of Light Historical Background
- Newton (1670s): Proposed corpuscular theory — light = tiny particles
- Huygens (1678): Proposed wave theory — light = waves in “ether”
- Young (1801): Double slit experiment proved wave nature (interference)
- Maxwell (1865): Proved light = electromagnetic wave
- Einstein (1905): Photoelectric effect proved light also has particle nature
For this chapter, we focus on Huygens’ wave theory which explains reflection, refraction, and forms the basis of wave optics.
Types of Wavefront
A wavefront is a surface connecting all points of a wave that are in the same phase.
Three types:
1. Spherical wavefront:
- From a point source in 3D space
- All points on a sphere centered at source are in same phase
- Rays are radially outward — not parallel
2. Cylindrical wavefront:
- From a line source (long narrow slit)
- Wavefronts are cylinders expanding outward
3. Plane wavefront:
- What a spherical wavefront becomes very far from the source
- All rays are parallel, wavefront is flat
- This is what we assume for most optics problems
- Lasers produce nearly perfect plane wavefronts
Huygens’ Principle Statement
Two parts:
- Every point on a given wavefront acts as a source of secondary spherical wavelets that spread forward with the same speed as the primary wave
- The new wavefront at any later time is the common tangent (envelope) drawn to all the secondary wavelets
This principle can construct the wavefront at any future time if you know the present wavefront.
Law of Reflection Using Huygens’ Principle
When a plane wavefront AB falls on a plane mirror:
- Point A touches mirror first and sends secondary wavelet
- By the time wavelet from A has radius r = vt, point B just touches mirror
- The reflected wavefront A’B’ is the common tangent to all wavelets
From geometry: Angle of incident wavefront with mirror = Angle of reflected wavefront with mirror → ∠i = ∠r (Law of Reflection proved)
This proof is a direct NEB exam short-answer question.
Law of Refraction (Snell’s Law) Using Huygens’ Principle
When a plane wavefront passes from medium 1 (speed v₁) to medium 2 (speed v₂):
- In the same time t, wavelet from A travels distance v₁t in medium 1 but point B’s wavelet travels v₂t in medium 2
- Since v₁ ≠ v₂, the new wavefront tilts
From geometry: sin i / sin r = v₁/v₂ = n₂/n₁
This is exactly Snell’s Law — proved from first principles using Huygens’ theory.
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