Discuss the below:
Q: 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?
2) Why is the contrast diminished for the sodium lamp?