In the figure below, a broad beam of light of wavelength 560 nm is sent directly
ID: 1906922 • Letter: I
Question
In the figure below, a broad beam of light of wavelength 560 nm is sent directly downward through the top plate of a pair of glass plates, which are shown in cross section touching at the left end. The air between the plates acts as a thin film, and an interference pattern can be seen from above the plates. Initially, a dark fringe lies at the left end, a bright fringe lies at the right end, and nine dark fringes lie between those two end fringes. The plates are then very gradually squeezed together at a constant rate to decrease the angle between them. As a result the fringe at the right side changes between being bright to being dark every 13.0 s.Explanation / Answer
= 560 nm, t = 13 s, d = spacing between the plates at the right end
a) th e end is bright: 2d1 = k
the end is next dark: 2d2 = k - /2
spaceing change d1 - d2 = /4
change rate = (d1 - d2)/t = /(4t) = 1.08*10-8 m/s
b) Initially, a dark fringe lies at the left end, a bright fringe lies at the right end, and nine dark fringes lie between those two end fringes.
2d3 + /2 = (9 + /2) + /2, d3 = 19/4
Later, both left and right ends have a dark fringe and there are five dark fringes between them:
2d4 + /2 = 6 + /2, d4 = 3
change = d4 - d3 = -7/4 = -9.8*10-7 m