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What is fringe width.

What is fringe width in the interference of light using double slit experiment.

I thought it was the width os a single bright fringe, but when I did this online experiment:

http://www.matter.org.uk/schools/Content/Interference/doubleslits_1.html

My answers didnt came correct. ( answers on http://www.matter.org.uk/schools/Content/Interference/wavelengths.html)

I rechecked and it seems that if I double the value I measured for fringe width the

answers come correct.

Is the fringe width the sum of the width of a dark and bright fringe? or is fringe width the length

between two consecutive fringes (from centre)? I think it has to be one of them.