Why is 30% recommended as the minimum michelson contrast?

Explained

Explanation:

Michelson contrast is used for patterns where the distribution of bright and dark segments is nearly equal.

It is given by:

where I_max = maximum illumination and I_min = minimum illumination

we know that

typically, I_min = 54% of I_max  (general standard)

or I_min = 0.54 I_max

putting this value in above equation to get m

this approximately corresponds to m = 0.3 or 30%

hence, 30% recommended as the minimum Michelson contrast

a.) travels faster in solids because the particles are closer together.

air is a gas water is a liquid