A total of 42 bright and 42 dark Newtons rings not counting

A total of 42 bright and 42 dark Newton\'s rings (not counting the dark spot at the center) are observed when 550 nm light falls normally on a planoconvex lens resting on a flat glass surface (Fig. 24-31). How much thicker is the center than the edges? mu m

Solution

The optical path length difference is = 2 h_m + lambda/2 = (m + 1) / 2 lambda

So the thickness of the air film at the edge can be obtained as

h_max = m_max * lambda / 2

= 42 * 550 nm / 2

= 11.55 mu.m