CBSE Class 12 Physics Notes Chapter 10 Wave Optics

Light interferes, diffracts, and it even undergoes polarization. The branch of physics that deals with the wave nature of light is called “Wave Optics”. Different theories have been given by different physicists regarding the nature of light. Some of these theories are:

• Newton’s Corpuscular Theory
• Huygens’ Wave Theory
• Maxwell’s Electromagnetic Wave Theory
• Einstein’s Quantum Theory
• de-Broglie's Theory

Wave optics, also known as Physical optics is an intermediate method between geometric optics, which ignores wave effects. CBSE Class 12 Physics Notes for Chapter 10 Wave Optics are given in the article below for easy preparation and understanding of the concepts involved.

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Class 12 Physics Chapter 10 Notes - Wave Optics

Wavefront

• The locus of all particles in a medium, vibrating in the same phase is called Wavefront.
• It is suggested by Huygens.
• The direction of propagation of light is perpendicular to the wavefront.
• All points on a wavefront act as a source of secondary wavelets.

Types of Wavefront

• Spherical Wavefront (SWF): Formed by point sources.
• Cylindrical Wavefront (CWF): Formed by line sources.
• Plane Wavefront (PWF): Formed by point sources at a large distance.

Types of Wavefront

Huygens’ Principle

• Each source of light is a centre of disturbance.
• All particles equidistant from the source and vibrating in the same phase are known as Wavefront.
• Every point on a wavefront is a source of secondary wavelets.
• Huygens’ Principle gives the geometrical details of the traveling of a wave.

Huygens’ Principle

Laws of Refraction based on Huygens’ Principle

• Let AB be a plane wavefront incident on the boundary XY separating two media.
• The position of wavefront after time t is given the wavefront CD.
• Time taken by wavelets to reach B to C and from A to D is given by

t = BC/v₁ = AD/v₂

• CD will be a true refracted wavefront if the secondary wavelets from point P take the same time to reach point R.
• Considering these conditions, we can prove the laws of refraction i.e.

sin i / sin r = μ

Laws of Refraction

Laws of Reflection based on Huygens’ Principle

• Let AB be a plane wavefront incident on the boundary XY separating two media.
• The position of wavefront after time t is given the wavefront CD.
• The time taken by wavelets to reach B to C and from A to D is given by

t = BC/v = AD/v

• By considering these conditions, we can prove the Laws of Reflection i.e.

∠ i = ∠ r

Laws of Reflection

Coherent Sources of Light

• Sources of light having constant or zero phase difference are said to be coherent.
• Generally, obtained from a single parent source of light.
• Capable of emitting light of the same frequency or wavelength.
• They can emit light of almost the same amplitude.

Interference of Light

• The phenomenon of redistribution of light energy due to the superposition of light waves from coherent sources is known as Interference of Light.
• Constructive interference occurs when two light waves of the same phase superimpose.
• In this case, the amplitude and Intensity of the resultant wave is maximum.
• Destructive interference occurs when two light waves of opposite phases superimpose.
• In this case, the amplitude and intensity of the resultant wave is minimum.

Young’s Double Slit Experiment

• Monochromatic light falling on two narrow slits S₁ and S₂ acts as two coherent sources.
• When light coming from these slits is superimposed, an interference pattern will be obtained.
• In YDSE alternate bright and dark bands are obtained on the screen.
• These bands are called Fringes.

Young’s Double Slit Experiment

Path Difference

• Formula:

Δx = yd/D

Where

• Δx  is the path difference.
• y is the position of the fringe from the central bright fringe
• d is the distance between the two slits
• D is the distance between slits and the screen

Fringe Width

• The distance between any two successive fringes is called the fringe width.
• Formula:

β = λD/d

Where

• β is the fringe width.
• λ is the wavelength
• d is the distance between the two slits
• D is the distance between slits and the screen

Diffraction

• The phenomenon of bending of light around the corners of an obstacle is called Diffraction.
• It is characteristic of all types of waves.
• The greater the wavelength of of wave, the higher the degree of diffraction.

Types of Diffraction

• Fresnel Diffraction: If either source or screen or both are at a finite distance from the diffracting device, the diffraction is called Fresnel Diffraction.
• Fraunhofer Diffraction: In this case, both sources and the screen are at an infinite distance from the diffracting device.

Diffraction of Light Due to a Single Slit

• Condition for Maxima:

θ = (m + 1/2) λ/d

Where

• θ is the angle giving the direction of mth order secondary maximum
• m = 1, 2, 3,.......

• Condition for Minima:

θ = mλ/d

Where

• θ is the angle giving the direction of mth order secondary minima
• m = 1, 2, 3,....…

Diffraction of Light Due to a Single Slit

Resolving Power

• The ability of an optical instrument to form distinctly separate images of the two points.
• It is the reciprocal of the Limit of Resolution.
• Resolving Power of Telescope:

R_T = D/1.22 λ

• Resolving Power of Microscope:

R_M = 2ƞ sin β / 1.22 λ

Polarization of Light

• The phenomenon of restricting the vibrations of light vectors in a perpendicular direction in a plane perpendicular to the direction of propagation of light is called polarization of light.
• Tourmaline crystal is used to polarize the light and is hence known as Polarizer.
• Polaroid is a device used to produce plane-polarized light.

Brewster’s Law

• According to Brewster’s law, the refractive index of the refracting medium is numerically equal to the tangent of the polarizing angle.
• Formula:

μ = tan i_B

Where

• μ is the refractive index
• i_B is the polarizing angle or Brewster’s angle.

There are Some important List Of Top Physics Questions On Wave Optics Asked In CBSE CLASS XII