Michelsons interferometer

optics,interference,interferometer - types,construction,working & applications

1. MICHELSONS INTERFEROMETER .INTERFERENCE .INTERFERENCE FRINGES .INTERFEROMETERS .MICHELSONS INTERFEROMETER .APPLICATIONS AND USES

2. . INTERFERENCE It is the phenomenon which is associated with the wave nature of light. The phenomena of modification of intensity of light obtained by the superposition of two or more beams of light is called as interference. If the resultant intensity is zero or less than we expect from the separate intensities ,we have destructive interference while if it is greater we have constructive Interference.

3. .INTERFERENCE FRINGES

4. Real fringe Can be intercepted on a screen placed anywhere in the vicinity of the interferometer without a condensing lens system. Virtual fringe Cannot be projected onto a screen without a condensing focusing system. In this case, rays do not converge.

5. Non-localized fringe . Exists everywhere . Result of point/line source Localized fringe - Observed over particular surface - Result of extended source

6. . INTERFEROMETERS An interferometer is an optical device which utilizes the effect of interference. Typically, it starts with some input beam, splits it into two separate beams with some kind of beam splitter (a partially transmissive mirror), possibly exposes some of these beams to some external influences (e.g. some length changes or refractive index changes in a transparent medium), and recombines the beams on another beam splitter.

7. . TYPES OF INTERFEROMETERS Mach– Zehnder Interferometer Michelson Interferometer Fabry –Perot Interferometer Sagnac Interferometer Common path Interferometer

8. . MICHELSONS INTERFEROMETER A Michelson interferometer is a tool to produce interference between two beam of light. It is the most common design for optical interferometry, and was invented by Albert Abraham Michelson. The Michelson interferometer produces interference fringes by splitting a beam of monochromatic light, such that one beam hits a fixed mirror and the other hits a movable mirror. When the reflected beams are combined, an interference pattern is formed. To create interference fringes on a detector, the paths have to be of different lengths, or composed of different materials. Michelson interferometers are relatively simple in operation, and possess the largest field of view for a specified wavelength. They also possess a relatively low temperature sensitivity.

9. (1852-1931) Albert Abraham Michelson A.A Michelson. Conjectured that if the speed of light relative to the hypothetical ether was indeed constant, then the Earth’s orbital velocity relative to this ether should exhibit observable relativistic effects. He devised and constructed an optical interferometer with which he presumed he would then be able to detect the relative motion of Earth against the static ether. That is , since Earth’s orbital velocity is approximately 107000 km/h, Michelson anticipated that his interferometer would exhibit measurable change over the course of a year due to the substantial difference In velocity relative to the static background ether.

10. Figure 1: A Michelson Interferometer

11. Figure 2: Schematic Representation of Michelson Interferometer

12. .CONSTRUCTION The main optical parts consist of two highly polished mirrors M 1 & M 2 and a plane-parallel plate of glass G 1 (compensator plate) & a beam splitter . Sometimes the rear side of the beam splitter is lightly silvered, so that light coming from the source is divided into (1)a reflected & (2) a transmitted beam of equal intensity. The mirror M 1 is fixed to a movable carriage and can be moved along the well machined ways or tracks .This slow and accurately controlled motion is accomplished by means of the screw . To obtain the interference fringes ,the mirrors M 1 & M 2 are made exactly perpendicular to each other . Even when this adjustments have been made , fringes will not be seen unless two important requirements are fulfilled. First the light must originate from an extended source and second, the light must in general be monochromatic or nearly so .

13. . WORKING We see from Figure 2 that light from a source strikes a partially silvered beam splitter , causing part of the beam to be transmitted toward a mirror, M 1 , fixed to a moveable carriage. The remainder of the beam is reflected from the beam splitter toward a fixed mirror, M 2 . A glass compensation plate along the path toward M 1 ensures that the two paths are effectively identical. The two beams, once reflected from M 1 and M 2 , re-converge at the Beam splitter. A precision micrometer screw is connected to the M 1 carriage and allows the optical path length along the M 1 branch to be altered over a few mm. Fine adjustment screws on M 2 allow fine tuning such that the beams from each branch can be overlapped at the viewing screen.

14. When recombined and aligned, the beams fall on the interferometer’s viewing screen where the resultant light exhibits interference effects dependent on the length differences between the two paths . If the distances from the beam splitter to mirrors M 1 and M 2 differ by distance d, then the total difference in the path lengths traveled by the two beams of light is 2d. When the distance 2d is equal to an integer multiple of the light’s wavelength, constructive interference is observed at the viewing screen as the crests of the two beams overlap, and a bright spot or ring is seen as the viewing screen. Mathematically this condition is described by the equation 2d = mλ Where λ is the wavelength of the light and m is an integer .

15. In actuality, due to differences between internal and external reflections at silvered surfaces, a phase reversal of one of beam may result, in which the distance 2d is a half integer multiple of the light wavelength, destructive interference is observed at the viewing screen. Mathematically this condition is described by the equation 2d = (m+1/2) λ

16. As d is increased new fringes appear at the center and the existing fringes move outwards, and finally move out of the field of view. For any value of d, the central fringe has the largest value of m.

17. 1. Measurement of wavelength of light Move one of the mirrors to a new position d’ so that the order of the fringe at the centre is changed from m o to m.

18. 2. Measurement of wavelength separation of a doublet ( λ 1 and λ 1 +  λ ) If the two fringe patterns coincide at the centre: (Concordance) The fringe pattern is very bright

19. Concordance

20. 2. Measurement of wavelength separation of a doublet ( λ 1 and λ 1 +  λ ) As d is increased p and q increase by different amounts, with When the bright fringes of λ 1 coincide with the dark fringes of λ 1 +  λ , and vice-versa and the fringe pattern is washed away ( Discordance ).

21. Discordance = ( q+1/2 )

22. 2. Measurement of wavelength separation of a doublet ( λ 1 and λ 1 +  λ ) - Δ  can be measured by increasing d 1 to d 2 so that the two sets of fringes, initially concordant, become discordant and are finally concordant again. - If p changes to p+n , and q changes to q+(n-1) we have concordant fringes again.

23. .APPLICATIONS AND USES The key applications of a Michelson interferometer are as follows: In the Michelson-Morley experiment, which led to the development of the special theory of relativity. In astronomical interferometry. In optical coherence tomography. In analyzing the upper atmosphere, by revealing temperatures and winds, and by measuring the Doppler widths and shifts in the spectra of airglow and aurora. As a component in the helioseismic and magnetic imager to study solar variability, and to illustrate the sun‘s interior, along with the many aspects of magnetic activity. For the detection of gravitational waves . As a tunable narrow band filter. As the core of Fourier transform spectroscopy.

24. LIGO - Laser Interferometer Gravitational Wave Observatory To detect Gravitational waves, one of the predictions of Einstein’s General Theory of Relativity Hanford Nuclear Reservation, Washington, Livingston, Louisiana Arm length: 4 Km Displacement Sensitivity: 10 -16 cm When Gravitational waves pass through the interferometer they will displace the mirrors!

25. THANK YOU VERY MUCH Success is the ability to go from one failure to another with no loss of enthusiasm. Sir Winston Churchill Created and Presented By: Prabhukrupa chinmaya Kumar M.Sc Physics 1 st year GITAM UNIVERSITY