Basic Principle of Interferometer
Interferometer is a precise instrument for flow visualization. The
variation of refractive index of the flowing gas with density is the
basic principle used in thses system. The principle of interference of
light waves is used. At a phase angle, the number of fringes are in
relation to change in density with respect to the zero fringe condition.
Description of Interferometer
The arrangement consists of the following:
- Two windows to form a test section
- A light source
- Two Lenses L1 and L2
- Two beam splitters B1 and B2
- Two mirrors M1 and M2
- A display Screen.
Operation of Interferometer
- Light rays from a source are collimated with a lens L1. That is, the
light rays become a parallel beam of rays when they come out of the
- This collimated light rays are then split by a beam splitter B1. The two beams traverse at right angles to each other.
- That is the beam splitter B1 makes half of the light to go to mirrir M1. The remaining half is reflected towards mirror M2.
- Beam 1 is made to pass through the test section (to experience the
flow filed) and beam 2 travels an alternate path, but of equal length.
- The two beams are agin brought together with the help of beam splitter S2 and are then focused onto the display screen.
- Because of the variation in the refractive properties of the flowing
gas in the test section, beams-1 will have a travel path of different
optical length when compared to that of beam 2. Beacuase of this, the
two beams will be out of phase and will interface when they are joined
together at B2. This causes alternate bright and dark regions called
fringes on the display screen.
- The number of fringes will be a function of the difference in the
optical length of the two beams. That is, for a difference in the path
lengths of one wavelength, one fringe will appear. For a difference in
the path lengths teo wave lengths, two fringes will appear and so on.
- It is to be noted that if medium I the test section has the same
optical properties as that of the medium experience by beam 2, no fringe
shifts will appear.
- Thus by observing ( and photographing) the interference effects,
direct measurement of density variations of the flow in the test section
can be obtained/ visualized.
- The only disadvantage in yusing this instrument is that it is
difficult to align the setup to get beams that have travel paths of the
same optical lengths.