Answer:
the thickness of the mica is 6.64μm
Step-by-step explanation:
By definition we know that the phase between two light waves that are traveling on different materials (in this case also two) is given by the equation

Where
L = Thickness
n = Index of refraction of each material

Our values are given as

Replacing our values at the previous equation we have


the thickness of the mica is 6.64μm