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# Central fringe physics !! Watch

1. A narrow beam of light is directed normally at a slit.
a) blue filter placed in the path of a beam before it reaches the slit. distance across 5 fringes including the central fringe = 18mm find the width of the central fringe
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Just quoting in Puddles the Monkey so she can move the thread if needed
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(Original post by Puddles the Monkey)
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4. (Original post by ashg99)
A narrow beam of light is directed normally at a slit.
a) blue filter placed in the path of a beam before it reaches the slit. distance across 5 fringes including the central fringe = 18mm find the width of the central fringe
The central fringe is double the width of the other fringes, so we can take the total width of 5 fringes including the central fringe to be equal to 6 "outside fringes"

18/6 = width of subsidiary fringes = 3mm

because central fringe = 2xsubsidary fringes
= 2*3 = 6mm

If you then add 3+3+6+3+3 (the widths of all the fringes in order, the answer = 18mm, so central fringe = 6mm

(I hope this makes sense, it did to me haha)
5. (Original post by jakepds)
The central fringe is double the width of the other fringes, so we can take the total width of 5 fringes including the central fringe to be equal to 6 "outside fringes"

18/6 = width of subsidiary fringes = 3mm

because central fringe = 2xsubsidary fringes
= 2*3 = 6mm

If you then add 3+3+6+3+3 (the widths of all the fringes in order, the answer = 18mm, so central fringe = 6mm

(I hope this makes sense, it did to me haha)
Thanks mate it did make sense. LoL

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