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Interference of Light: Contrast in Laser and Sodium Light

When a laser is used to illuminate a Michelson interferometer, we will see fringes with excellent contrast even when the two arms are a centimeter or more away from equal length. But when we use a sodium lamp as the light source, in place of the laser, fringes are visible only when we have extremely close to equal arm lengths. 1) What is the main reason for this difference? I only need a qualitative answer, so I can understand what is happening. 2) Why is the contrast diminished for the sodium lamp? Again qualitative answer is fine. I saw this in lab, but want to make sure I understand it.


Solution Summary

Why the contrast in the fringe pattern of laser and sodium light is different? Explained in detail.