Evans SPH 4U1

Physics Grade 12

Unit 5: Waves

Note 12: Michelson's Interferometer


Reference: Chapter 10.7


The interferometer relies on the principle:

When the path difference of two waves is half a wavelength (or any odd multiple of half a wavelength) apart the waves are out of phase.



Used to measure very small distances (i.e. wavelength of light)

Verified that speed of light is constant in a vacuum and for all frames of reference (to be discussed in relativity later).

Three mirrors are used (a fixed mirror, a movable mirror and a middle mirror).  A monochromatic light is split into two beams.  One beam travels to the fixed mirror and one beam travels to the movable mirror where they are reflected back to the middle mirror and back to the observer. If the fixed mirror and the movable mirror are the same distance apart there will be constructive interference.  If the movable mirror is moved then one beam will travel a total of farther and destructive interference will occur.  Therefore each fringe move (i.e. bright to bright, OR dark to dark) will be a mirror movement of .


Example 1:

An interferometer is illuminated by a monochromatic source of wavelength 6.4 x 10-7m. You slowly move the movable mirror. You find that 100 bright fringes move past the reference point. How far did you move the mirror? (3.2 x 10-5m)


Example 2:

An interferometer is illuminated with monochromatic light. When one of its mirrors is moved

2.00 x 10 -5m, 100 fringe pairs pass by. Find the wavelength of the incident light.

( Since each fringe move is then so the answer is )