The consideration of the passage of light waves through three media elucidates an interesting point in the Theory of Newton's Rings.

Let the three media be denoted by the numerals 1, 2, 3, and let z = 0 be the plane of separation between media 1 and 2 and z = −a the plane of separation between media 2 and 3. Also let the axis of x in the plane z = 0 be parallel to the wave front of the incident plane waves.