A two-slit pattern is viewed on a screen 1.17 m from the slits. What is the width of the central bright fringe, if the third dark fringes on either side are 26.4 cm apart?

Answer 1

The coordinate of dark fringe is
y=(2k+1)Lλ/2d.

The width of the central bright fringe= the distance between the first dark fringes is
2y₁=2•(2•1+1)Lλ/2d =3 Lλ/d.
The distance between the third dark fringes is
2y₃=2•(2•3+1)Lλ/2d=7 Lλ/d.
2y₁/2y₃=(3 Lλ/d)/( 7 Lλ/d),
2y₁/2y₃=3/7,
2y₁=2y₃•3/7=
=26.4•3/7=11.3 cm

Answer 2

Sorry not the right answer