A Fizeau interferometer is a device to judge flatness of a surface. Many optical instrumentation are tested using Fizeau interferometry.

1. FIZEAU INTERFEROMETER
SHUBHAM MAURYA

2. Principle
Reflected light from reference
plate and test flat produces
interference patterns of different
intensity
•I = I1+ I2 + 2√(I1I2) cos f
•The resultant intensity of
reflected light is used to judge
the nature of test surface.

3. Working and construction
 It comprises of a monochromatic light source
(e.g. laser) passed through a pin-hole using a
converging lens.
 The light then illuminates the objective, and
because pin hole was at focus of this lens,
light rays become parallel.
 Objective lens acts as collimator
 The parallel light emerging from collimator
passes through high quality reference flat
which is permanently built into the instrument.

4. Working and construction
(cont.)
 The reflected light from reference flat and test
flat undergo interference and pass through
beam splitter to the eyepiece or CCD.

5. Guidelines and modifications
 The reference flat is adjusted
till the image of pinhole
disappears from
screen/eyepiece.
 It is done so that reference flat
is perpendicular to the parallel
light beams.
 Reference flat is beveled (see
Fig.) so that light reflected from
that surface will not interfere
with light coming from test flat. Fig. Beveling of reference flat

6. Applications
 Extensively used for testing optical
components in space related instrumentation.
E.g. primary mirrors in Hubble Space
Telescope
 Checking optical flats, wedges, front surface
mirrors and glasses are major applications.

7. Applications
Fig. Testing spherical surface of the test lens with respect to reference spherical su

8. Applications
Fig. Fizeau interferometer for measuring the effect of water movement upon the speed
of light. The interference pattern can be analyzed to determine speed of light in each of
the tube. This is the famous Fizeau experiment whose unexpected results were
explained by Special Theory of Relativity.