WAVE OPTICS & LASER.

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Presentation transcript:

WAVE OPTICS & LASER

WAVE OPTICS & LASER Interference Air-Wedge – Theory and Applications Types of LASER Nd:YAG LASER, CO2 LASER, Semiconductor LASER (homojuntion) Application : Holography Fiber Optics – Principle Types of Optical Fibers Fiber Optics Communication System

WAVE OPTICS Wave Optics deals with different optical phenomena Interference Diffraction Polarisation

INTERFERENCE A phenomenon in which two waves superpose to form a resultant wave of greater amplitude or lower amplitude Interference effects can be observed with all types of waves Light waves Radio waves Acoustic waves Surface water waves Matter waves

Path Difference and Phase Difference Path difference and phase difference will be meaningful for waves of same frequency They are used to determine constructive and destructive interference in waves Path difference (measured in terms of wavelength) Actual measurable difference in distance that two waves travel from a source to a common point Path difference between the red and blue wave = λ/ 4 Phase difference (measured in radians) Difference in the phases of the two sinusoidal waves of same frequency The blue wave leads the red wave by a phase difference of π/2 (90 degrees)

Path Difference and Phase Difference At time t = 0 Blue wave displacement = 0 Red wave displacement = – A At π/2 Blue wave has maximum displacement = + A Red wave displacement = 0 Blue wave is leading by a phase difference of π/2 and path difference of λ/4 One oscillation is completed in 2π radians which is equivalent to wavelength λ (Path difference of one wavelength (λ) is equal to phase difference of 2π radians)

PD = 5λ – 4.5λ = 0.5λ = ½ λ PD = 5.5λ – 4.5λ = λ Waves follow different paths from the slits to a common point on a screen (a) Destructive interference One path is a half wavelength longer than the other The waves start in phase but arrive out of phase (b) Constructive interference One path is a whole wavelength longer than the other The waves start out and arrive in phase

Constructive Interference

OPTICAL INTERFERENCE Formation of bright and dark bands resulting from the super-position of two coherent light waves Constructive Interference (Bright band) Path difference = n λ Phase difference = n 2π Destructive Interference (Dark band) Path difference = (2n – 1) λ/2 Phase difference = (2n – 1)π Coherent light waves Light waves have same wavelength same amplitude constant phase difference

Optical Interference Band Width or Fringe Width Distance between two successive bright bands or dark bands

Optical Interference OPL = μ d To obtain clear and broad interference bands Wavelength of light should be larger Light sources should be closer and narrower Distance between the screen and the coherence sources should be larger OPTICAL PATH LENGTH Product of the geometric length (d) of the path light follows through a system and the refractive index (μ) of the medium through which it propagates OPL = μ d

Polarisation Optical phenomenon in which the light vibrations are restricted to take place in a single plane An ordinary light is unpolarised light since it can vibrate in all directions in a plane perpendicular to the direction of propagation of light

Interference at a Wedge-Shaped Film ABC - A wedge shaped film of refractive index (μ) with very small angle A parallel beam of monochromatic light is incident on the upper surface The surface is viewed by reflected light through a travelling microscope Alternate dark and bright bands can be observed

Interference at a Wedge-Shaped Film An air wedge is formed by two plates of glass separated at one end No phase change 180o phase change

Interference effect is between light reflected from the bottom surface of the top plate and light reflected from the top surface of the lower plate Bottom of the upper plate → glass-to-air boundary (μglass > μair) No phase change upon reflection Top of the lower surface → air-to-glass boundary (μair < μglass) There is a 180° phase change upon reflection from this surface

Interference at a Wedge-Shaped Film Point P will appear dark and a dark band will be observed across the wedge, if Point P will appear bright and bright band will be observed across the wedge, if

Interference at a Wedge-Shaped Film If the nth dark fringe is formed at P Similarly, for the (n+1)th dark band which is formed at Q at a distance x2 from A

Interference at a Wedge-Shaped Film Fringe width (β) So, if we consider any two consecutive bright fringes, β will be the same

Interference at a Wedge-Shaped Film A wedge shaped air film can be obtained by inserting a thin piece of paper or hair or thin wire between two glass plates For air film t – thickness of paper or hair or thin wire x – distance from the edge to the paper or hair or thin wire

Thickness of a paper or wire or hair A wedge shaped film is obtained by inserting a thin paper or thin wire or hair between two parallel glass plates (optical flats) Sodium vapour lamp is used as the light source Light is incident on the glass plate inclined at 45⁰ to the horizontal Glass plate makes the light to fall normally on the optical flats by reflection By adjusting the distance between the microscope objective and optical flats, we can get the interference fringes in the microscope eye piece

Thickness of a paper or wire or hair By coinciding one of the bright fringes with the vertical crosswire, the reading is noted in the horizontal scale of microscope After crossing of five bright fringes, once again the reading is noted The value of fringe width is calculated Distance “x” from the edge where the two plates touch each other to the paper or thin wire is measured Then the thickness t can be calculated from the formula

Testing of flatness of a surface

Testing of flatness of a surface Not smooth surface Incident light Smooth surface Incident light

Bright Band Path Diff = nλ Phase Diff = n2π Dark Band Path Diff = (2n-1)λ/2 Phase Diff = (2n-1)π Interference Wave Optics Thickness of thin sheet or wire or hair Testing of Flatness of a surface Air Wedge shaped film