PPT Slide
1. Find d for the 19th and 20th bright fringe:
path difference? black ray travels extra distance 2d in air =>phase diff = (2?/?)(2d) note: n=1!
black ray has extra phase difference of ? due to reflection
bright fringe when ? = (2?/?)(2d)+?=2?m => d=(m-1/2)(?/2)
d19 = (19 – 1/2) ? /2 = 5457 nm; d20 = 5753 nm
hence 5.46 µm < d < 5.75 µm
The diameters of fine wires can be accurately measured using interference patterns. Two
optically flat pieces of glass of length L are arranged with the wire between them as shown above.
The setup is illuminated by monochromatic light, and the resulting interference fringes are detected.
Suppose L = 20 cm and yellow sodium light (?? 590 nm) is used for illumination. If 19 bright fringes
are seen along this 20-cm distance, what are the limits on the diameter of the wire?
Hint: The nineteenth fringe might not be right at the end, but you do not see a twentieth fringe at all.