Interference pattern is obtained with a source of red light of wavelength 6400 Å. The source is then replaced by another source of wavelength 4000 Å. It is found that nth bright fringe of first source coincides with ` ( n + 3 ) ^( th ) ` bright finge of second source. The order of finge is equal to

Options

A. 6
B. 5
C. 4
D. 3

Answer

B

Explanation

According to the question
` ( nDlamda _ 1 ) /( d ) = ( ( n + 3 ) D lamda _ 2 ) /( d) `
` rArr n ( 6400 ) = ( n + 3 ) 4000 `
or ` 1 ` 6n = ` n + 3 or 0" " 6n = 3 `
` rArr n = 5 `